Garry's Mod

Garry's Mod (commonly abbreviated as GMod) is a sandbox physics game developed by Facepunch Studios and published by Valve Corporation. Garry's Mod was originally a mod created by Garry Newman for Valve Corporation's Half-Life 2 but was later made into a standalone release in 2006 for Microsoft Windows. Later updates saw an OS X port, added in 2010, and a version for Linux in 2013.

Gameplay
The base game mode "sandbox" gives the player the freedom to spawn non-player characters (NPCs), ragdolls or objects (called props), such as furniture, shipping containers, dumpsters and explosions, and interact with them in various ways.

Avariety of props, NPCs, and ragdolls can be selected and placed into the sandbox from any installed game running on the Source game engine or from the 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 (sometimes called the Phys-Gun), allows the 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 accessed through the Steam Workshop.

Engine
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 realistic simulations of structures and experiments.