Since GMod, short for Garry's Mod, is a sandbox game, players can play is however they wish; however, user-submitted modifications, or mods, to Garry's Mod often have their own sets of rules. In the default sandbox mode, players can create and interact with items and the landscape itself.
Garry's Mod began as a mod to Half-Life 2 but was later released as a standalone game. The game provides a number of tools to allow players to create their own world and interact with it, and user guides typically cover how to experiment with the tools the program provides.
However, much of the popularity of Garry's Mod is due to its user community. With six million sales as of 2013, the program has a large user base and a number of people who use Garry's Mod to create their own games, scenarios or showcases. Players can log in to servers that host these mods and interact with other players. Mods range from fairly traditional games to bizarre demonstrations.
Garry's Mod comes with a programming interface that allows users to change the program significantly without having to delve into its code base. As a result, new game designers sometimes use Garry's Mod as a tool for experimenting with creating programs and testing new ideas.