Garry's Mod is a 2006 sandbox game developed by Facepunch Studios and published by Valve. 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. 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. 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. Ports of the original Microsoft Windows version for Mac OS X and Linux followed in September 2010 and June 2013, respectively. As of September 2021, Garry's Mod has sold more than 20 million copies.
-some random gmod info website