Although Garry’s Mod is listed as a full game, the base game mode ‘sandbox’ has no set objectives and instead gives the player the freedom to manipulate rag dolls or props such as furniture, shipping containers and dumpsters.
Props and ragdolls can be selected and placed into the sandbox from any installed Source engine game or from community created collections, such as PHX3 for props and Civil Protection model packs for ragdolls. An important tool offered to the player, the Physics Gun allows for props and ragdolls to be picked up, rotated, and frozen in place. Another important implement, the Tool Gun is a multi-purpose tool for performing various tasks, such as constraining props together, creating interactive buttons, and creating controllable winches and wheels. It can also be used to change the facial expression and pose the digits of a ragdoll. The Tool Gun can also be used to control and use add-ons created by the community, which can be only accessed through the Steam Workshop.
The game uses the Source engine’s modified version of the Havok Physics Engine, which allows players to build contraptions that follow the laws of physics, allowing for very real simulations of structures and experiments.