Garry's Mod is a physics sandbox. Unlike regular games there aren't any predefined aims or goals. We give you the tools and leave you to play. You spawn objects and weld them together to create your own contraptions, whether that's a car, a rocket, a catapult or something that doesn't have a name yet, that's up to you.
This game requires that you own a Source engine game such as Counter-Strike: Source, Half-Life 2, or Day of Defeat: Source.