In Garry's Mod, everything (or almost everything) is possible. At Steam's workshop, it exists every type of mode: guns, NPCs/PCs from other games, maps, tons of ragdolls, game modes, vehicles, and much much more, all made by a creative community. I highly recommend this game, especially to play with friends. You can play a specific game mode or simply go for sandbox and make a big mess there, it is up to you!