This game is not what you usually see on Steam. In Garry's Mod, you enter a physics sandbox which provides you with tools to make what you want happen and leaves you to it. You can play alone or join thousands of other players kidding around and playing with physics. If you do try this game, do check out its Steam Community Workshop

Garry's Mod or GMod (stylized as garry's mod) is a sandbox physics game using a modified Source engine, created by Garry Newman. Garry's Mod was originally a modification, a "mod", for Valve's Half-Life 2, but was later made into a standalone game. I ...   more on Wikipedia