Garry's Mod

Garry's Mod (unofficially abbreviated as GMod) is a open-world physics game that allows the player to manipulate objects and experiment with physics. Garry's Mod has been available on Steam's content delivery service since November 29, 2006. Even though Garry's Mod is a standalone game, it requires the user to own at least one Source Engine game on Steam such as Half-Life 2, Portal, Team Fortress 2, Counter-Strike: Source, Left 4 Dead, or Left 4 Dead 2.