This is gonna be a long post but I want to talk about the more unique aspects of this game that aren't the game mechanics like the emotion system it has, how unique and impressive the AI is and so on that I've already seen people talk about. Most people into "strange" PS1-era video games have heard of this one before. It is a bit of an infamous title due to still being the only video game ever to win the Seiun Sci-fi award and at the time it was just made by a bunch of random staff that somehow made a mecha-focused game leagues above any of its competitors such as Front Mission, SRW, Sakura Wars and so on. It is also sometimes mentioned in relation to Muv-Luv and 13 Sentinels. The former has basically the same alternate-history concept and the way it handles a certain SF trope is very similar but otherwise they are quite different. With the latter, it is probably the primary influence (alongside Please Save My Earth and Megazone 23) which is pretty hilarious as one of the most common complaints with 13S is that it "doesn't do anything original" despite the fact that 90% of the people who say that don't even know what those three series are.
The influence of GPM on 13S isn't a simple plot point or anything like that though and much deeper than you would expect. Take 13S and remove the mystery files and imagine if every story in the story section was totally randomized. Not only that, but imagine the gameplay of 13S except it's not piss easy and it's incredibly complicated. But enough about other games, GPM has had a pretty legendary status as a long untranslated game. I have played through the game four times including the tutorial playthrough and it has been nearly a year.

First off, similar to 13S and Persona 3 the game is split into two sections being the social sections and the battle system. A good 90% of the game takes place in the school section while the rest of it is "SRPG" stuff. This may not sound like anything special but, if you ever wanted the social sections to tie into the battles in those previously mentioned games this is what you are looking for. Pretty much everything you do in the social sections matter, and each playthrough will be a different experience. The games social-link equivalent does not give you simple stat boosts or anything like that, almost every single classmate you have gives you entire game mechanics. Ranging from being able to see into peoples hearts and seeing who likes who to being able to use the Shining Finger from G Gundam, to being able to increase your social status and forcing your classmates to listen to you. Not only that, but there is perma-death. This isn't some FE style perma-death though. When a classmate dies, they hold an entire funeral for him and the way all the other characters act changes. Every single death matters. If you liked the aspect of not being able to control your classmates in Persona 3's battle section well, this is like that but a million times more complicated. The more someone likes you, the more likelier they are to help you during battle. This isn't all however, there are as many negative aspects as there are positive aspects to it. So if you are thinking of trying to be best friends with literally everyone, I learned the hard way that this is not a viable strategy. Most of the cast has beef with each other so you have to experience and choose who you want to get close to. For example, Katou is very useful and charming but getting close to her makes Kariya hate you because he feels jealous of your character. Kariya is disabled and on a wheelchair who needs help a lot of the time. Helping him up the stairs and stuff would normally make others sympathize with you more and see you as a helpful person, but if he hates you you lose this ability. On the other hand, getting close to a psychopath like Akane puts a rift between you and the rest of the cast. Moreover, the game is filled with beginners traps. If you get close to Mai Shibamura, the "main heroine" of the game you lose access to pretty much everyone else as they all hate her. The tradeoff is that Mai is easily the best character in the game and simply better than everyone else, despite this fact though if you get close to her it also locks you into one of the few scripted battles that a beginner has next to no chance of winning. There's something like this with every single character which is what makes it stand out so much to me. You have next to no control over how the world develops in the game, and you have to manually learn who likes who and vice versa by talking to them.

However, the more you play through the game the more you acquire this "knowledge" which leads you to be able to control the world more. This in turn is a requirement for the true ending in game. Once you fullfill the true ending requirements and "learn about the truth of the world", you start being referred to as a god and all your classmates start being afraid of you. This is another very unique aspect of the game, counting things like scripted bad endings there are around a dozen endings in the game. There are six normal endings depending on how you perform as a soldier, however besides the true ending most of the endings are the same besides the boosters they give for your next playthrough. The true ending though, is anything but simple. Pretty much the entire dialogue in the game changes once you fulfill the requirements for it. The requirements for it are harsh as well, you have to kill over 300 Genjuu (the beta/kaiju/alien equivalent of the game) and make sure no one among the 23 characters in the cast (yes, including your teachers) dies.
Oh yeah, the story. If you dislike long stretches of plot infodumps that RPGs tend to have, this is your game. Possibly the epitome of show don't tell. There is an overarching story but it takes a background to the point of the game, which is that the way you connect the strange happenings of the game is the "real story". Essentially, your experience while playing it matters more than the explanation to things. That isn't to say they don't explain anything but without spoiling much this game is unpredictable to say the least. Even with game mechanics, I've never seen any other mecha SRPG have war-time tax on simple items in the store. You have to pay fucking tax in this game. Simply put, this isn't a game that revolves around twists. The amount of stuff you can do is virtually limitless, if you want to become the commander of your platoon you can do it. If you think the battle system is bad, you can take a job that doesn't involve piloting such as being an engineer and sit out and skip all of the battles. Hell, if you want you can NEET it out too, not working at all and being lazy is an option as well. Planning an assassination against government officials, working for the black market, learning the origin behind the robots you pilot, "siding with the aliens", getting real close to the cat that suspiciously has god-tier stats, cursing your classmates and setting them up to die, working out like crazy and punching aliens while wearing your piloting suit and so on. All this stuff I mentioned isn't even half of it, I recommend not looking up many guides as learning that every single action has a consequence is one of the things I liked about the game. Oh yeah, there is an "Illusion" meter in the game that is basically a schizophrenia stat. You should absolutely grind that to see some wild shit.
So in the end how does all of this come together? Well, barely. The music is very underwhelming. The visuals of the games besides the battles are in the negatives. A lot of the character scenarios feel unfinished, and it doesn't help that for some reason all the social link equivalents are in randomized order for the most part. I would say the game is complete as it is but, if you go into it expecting a set beginning and an end you will be disappointed. A lot of stuff was cut as well, going into minor spoiler territory here there are 5+1(the tutorial chara) playable characters. Originally, every single character was meant to be playable. You can still play as every character through a cheat code but all of their scenarios are unfinished and insanely glitchy. Moreover, all the ED cutscenes have a bunch of CGs that you don't see in game: all content that was cut and reproduced as an audio drama. Said audio drama is largely unavailable, I had to physically buy most of the discs and uploaded what I have on YT but I'm still missing the last two.
There's a ton of stuff about the game that I haven't mentioned due to spoilers. Some crucial game mechanics, the meta element that the entire game revolves around, etc. One final note is that if you're the type that lacks reading comprehension and whines about Muv-Luv and similar games being fascist then steer clear of this game because it makes ML nationalism seem like a joke.
Gunparade March lived up to the hype for me and I still think about it every day but, I feel like a lot of people won't really like it much. It really is a lightning in a bottle tier game. I'm hoping LOOP8 will be good but it isn't looking that great and the other sequels are not great. Still, if you like any of the aforementioned games or if anything I said about it sounds interesting it is absolutely worth checking out in the case that it gets translated. Just know that whatever the game seems like to you, it's probably nothing like that in reality.

Added and corrected. 2021/10/11
The quoted part of the lesson has been added and corrected without omission. And added 'on military song'. 2022/02/27

A small miracle by a small company in Kumamoto, Kyushu.
"Where is Kyushu? Oh, on the edge of Japan. Very rural."
Shut up!
The name of the miracle is Gunparade March.

Desperate war situation. Student soldiers. Militaries. Robots. Simulation.
This is a highly recommended game for those who like this kind of word.

A game so geeky it's almost forgotten, but this one is really important.
Without the Gunparade March, there would have been no Muv-Luv, and without Muv-Luv, there would have been no Attack on Titan. Does this help you to understand the importance of the matter?
Oh yes, and by the way, the 13 sentinels.

It may seem old-fashioned and inconvenient to play now. But even with all those negatives, Gunparade March had more than enough sparkle and ambition.

There are 22 controllable student characters. Three complete NPCs and a cat.
These 25 people and one cat will move around the school, which has become a garrison, using AI control to think independently, and will study, train and work every day. If you get to know each character, you can even fall in love with them, and they will teach you tactical commands that will help you in battle and wisdom that you can use in everyday life.
The field you can go to is not wide. But the relationships between these 25 people are deep.

The number of active days in the game is about two months. During these two months, as a new soldier, you will be fighting against monstrous enemies called Phantom Beasts.
If you fight hard and get results, the front will advance and your troops will be better supplied. In return, you'll be sent to more and more battlefields, and the fighting will get more intense... (Naturally, the longer you lose, the worse your supply gets.)
It is up to you to decide how this game world will end.

I was also impressed by the great enthusiasm of the text.
I will close this review with some English translations of some of the texts.

The first lesson after the start of the game.

The first lesson after the start of the game.
*Teacher: "Lesson one.
Don't be late. In the army, timing is everything. If you're late, you'll never be able to coordinate your forces. They will be destroyed individually.
If a hundred men come at you at once, you can't defeat them alone.
But if each of those 100 men comes in five minutes late, you, alone, can do quite well.
It's not one against one hundred, it's one to one a hundred times.
That's what I'm talking about. So don't be late.
I'm not saying don't be late without reason, like in a bad school.
You and your friends' lives depend on it.
Civilian teachers just can't understand this.
...OK, that's it, orientation over.
Now it's time for class. You don't have to take notes.
Make sure you remember what I say.
You won't have time to read your notes on the battlefield.
Now, students.
Kaka, that's not like me. Hey, samurai.
You're samurai.
You take up arms and fight to defend this country.
In other countries, we call them soldiers. In this country, we have a better name for it.
I don't know if you wanted to be a soldier, or if you didn't want to be a soldier, but you were drafted, or if you replaced your brother.
But you're samurai.
That's why you came here without refusing to be drafted.
You are not samurai because of your ability.
At least, not in this day and age.
The heart that takes up arms for someone else is samurai.
Now, Kumamoto has always been a fine soldier, from the time when this country was westernized, it has sent out the strongest divisions along with Tohoku for generations.
You are their youngest brothers.
You should be proud of yourselves. You should be proud that you are the descendants of the strongest soldiers.
Protagonist: "Sir, I'm(Boku) not from Kumamoto."
Teacher: "If you're a tanker, don't refer to yourself as 'I/僕/Boku'. If you are dealing with a superior, you must say 'Myself/自分/Jibun'.
So, you're not from Kumamoto? Hmmm.
I've been thinking about it since yesterday...
Well, okay. I'm not just talking about your hometown.
I want you to have confidence in yourselves.
Lesson 2.
A strong human on the battlefield is, in short, a human who knows how to use itself.
You've got to have confidence in yourself to use yourself.
Otherwise, you can't use yourself to your full potential, can you?
If guy and bitch aren't sure of yourself, in a pinch you'll abandon your friends and run away. Without demonstrating his ability.
That's why we give new and student soldier confidence and pride for now.
We are brave, we are the last shield that protects this country, the tip of the sword of this country.
They said there was nothing more behind them.
It is weak, but it is always the last thing on your mind when you plunge your feet into a pool of blood, go under a storm of shells and trample the corpses of your brothers-in-arms.
In any army, there is one thing that you ultimately trust. It's people and their loyalty.
When the economy collapses, when the weapons are destroyed, when the enemy is strong, when we are about to lose, I know, what I can trust.
It's not wads of cash, it's not weapons.
In times of trouble, it's the heart that ultimately comes to the rescue. The heart is the only thing that will not betray a good memory.
It's the same for individuals and for countries.
It is the good memories of each and every one of you, of this country, that the army will turn to at the last moment.
If they are not good, this country will perish.
If you have a few good memories, you will attract samurai. Young samurai who will protect this country.”*

The heroine's(The official description is Hero.) words after the above text.
*"Do you know what that is? That kind of lesson is called mind control.
...It's the government's elaborate job.
...Well, all right."*

Likewise, The heroine's words in the game's most intense battle.
*"For us in Shibamura, this battlefield is like home. We were born in the battle and we die in the battle.
I don't like it, but it's hometown.
It's our hometown, where our hearts are warmed.
We have found friends, lovers and trust here.
The path we choose is the path of battle. There is a pride that we must defend in battle and in death.
For the sake of all, we will fight in the front row."*

This is a line from a boss who doesn't get much screen time but has a strong presence.
”I don't like to waste soldiers. People should die more efficiently."

On military songs. An extra-curricular lesson by another teacher.
*Teacher: "...In times of distress, sing a song.
Singing is the only sacred and unspoken right of a soldier whose profession is to kill each other on command.
You can sing even if your hands are cut off or blocked. Officer can't tell you not to sing.
Because he sings too.
In times of distress, in times of sorrow, in times when you need to lift yourself up, the only thing that can help you is singing.
It's the Gunparade March.
'...A thousand million of you and me, we'll beat that fate. Let's sing a march for the future of someone somewhere. Yes, the future will always be with this march.'
Take up a gun for someone else's future.
Because if there is any meaning to this stupid war, it is that."*

It's a very quirky game, but I highly recommend it.