The gameplay of "Garry's Mod" involves players manipulating and modifying objects in the game world at their leisure. As such, there are no true rules for how the game is played, though certain etiquette rules are in place to ensure a positive experience for all players.
One of the only rules of using "Garry's Mod" is to not hack, modify or exploit parts of the game to hinder the enjoyment of the game for other players. This is called "griefing," and typically consists of creating mods to interfere with other players intentionally, often in a way that is uncontrollable. The moderators of "Garry's Mod" have the ability to suspend or ban players believed to be griefing other users.
Other than rules meant to enforce etiquette, "Garry's Mod" is entirely rule-free, with players able to play the game however they choose. The developers at Facepunch Studios specifically designed "Garry's Mod" so as to have few rules. "Garry's Mod" began as a mod for the PC game "Half-Life 2" in 2004. It was released as its own game on Steam in 2006. As of July 2013, "Garry's Mod" had sold over 3 million copies, with numerous new updates having been added over time.