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. Gameplay The player character (right) positioning characters from Team Fortress 2 on a couch using the physics gun As of September 2021, Garry's Mod has sold more than 20 million copies. Ports of the original Windows version for Mac OS X and Linux followed in September 2010 and June 2013, respectively. Garry's Mod was created by Garry Newman as a mod for Valve's Source game engine and released in December 2004, before being expanded into a standalone release that was published by Valve in November 2006. Other game modes, notably Trouble in Terrorist Town and Prop Hunt, are created by other developers as mods and are installed separately, by means such as the Steam Workshop. The base game mode of Garry's Mod has no set objectives and provides the player with a world in which to freely manipulate objects. Garry's Mod is a 2006 sandbox game developed by Facepunch Studios and published by Valve.

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