Garry's Mod (officially abbreviated as GMod) is a sandbox physics game using the Source engine. Garry's Mod has been available on Valve's content delivery service, Steam, since November 29, 2006. Despite being a standalone game, Garry's Mod requires the user to own at least one game on Steam based on the Source engine, such as Half-Life 2, Counter-Strike: Source, Portal, or Team Fortress 2. It is also commonly used for the creation and publishing of videos on YouTube, mainly from Team Fortress 2, Half-Life 2, Portal, Left 4 Dead and Left 4 Dead 2. Garry's Mod was originally a modification for Valve's Half-Life 2, but was later made into a Steam game that was officially released on November 29, 2006.

Although Garry's Mod is usually considered to be a full game, it has no game objective and players can use the game's set of tools for any purpose. Garry's Mod allows players to manipulate "props" – various objects that players can place in-game. Props can be selected from any installed Source engine game or from a community created collection. The game features two "guns" – Physics Gun and Tool Gun for manipulating objects. The Physics Gun allows objects to be picked up, adjusted and frozen in place. The Tool Gun is a multi-purpose tool for performing various tasks, such as combining props, attaching them via ropes, and creating controllable winches and wheels. The Tool Gun is also used to control add-ons created by the community.