"Garry's Mod" is a sandbox game for Windows, OS X and Linux that offers abundant freedom to the player and has no objectives or restrictions. It is mostly centered around building, as players can create a host of structures and objects, including cars and rockets.
"Garry's Mod" offers players the choice of playing single-player or multiplayer game modes, the latter of which allows players to join friends in building cooperatively. In addition to the open-world game mode in "Garry's Mod," players can also opt for more restrictive play styles, such as hunting for terrorists, playing sports, running jails and many others. The community of "Garry's Mod" players has created a large number of modifications and add-ons that further enhance the game. "Garry's Mod" also offers the ability to pose and manipulate characters and props from other Source engine games the player has installed.
The physics engine in "Garry's Mod" is based on a modified version of the Havok physics engine. This modified version is native to the Source engine and is capable of emulating realistic physics, further aiding the structures and devices players create. Although "Garry's Mod" was initially a free game, it has sold more than 6 million copies, as of September 2014.