I have heard this from word of the mouth, from people talking about this type of mod when hosting a dedicated game server. What is Garry's Mod? Is it something you download from a .zip file or what?
Garry's Mod or GMod is a sandbox mod for the Source Engine. Unlike normal games there aren't any predefined aims or goals. Players are given tools and are left to entertain themselves. It is a physics sandbox game that allows the player to manipulate objects and experiment with physics.
You can find more information at: http: // en . wikipedia.org/wiki/Garry%27s_Mod