In late 2009, Facepunch launched the 'Fretta Contest', a competition in which people were to develop Garry's Mod game modes using the proprietary Fretta programming framework, with the winning game mode to be added to the base game. Fretta Contest and Trouble in Terrorist Town This was replaced by support for the Steam Workshop in version 13.

Garry's Mod version 12 introduced the 'Toybox' section, through which the player could browse and install user-created mods. Specialised servers, known as Fretta servers, rotate between custom game modes every fifteen minutes. Notable mods (known as 'addons') include Spacebuild, Wiremod, Elevator: Source, DarkRP, Prop Hunt, and Trouble in Terrorist Town. Garry's Mod includes the functionality to modify the game by developing scripts written in the Lua programming language. The 'tool gun' is a multi-purpose item for tasks such as welding and constraining props together, and altering the facial expressions of ragdolls. The individual limbs of ragdolls can also be manipulated. Using the 'physics gun', ragdolls and props can be picked up, rotated, and frozen in place.

The player is able to spawn non-player characters, ragdolls, and props, and interact with them by various means. Garry's Mod is a physics-based sandbox game that, in its base game mode, has no set objectives. The player character (right) positioning characters from Team Fortress 2 on a couch using the physics gun

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