Garry's Mod is a physics sandbox game which started out as a mod for Half Life 2. In the version 9 release, Lua scripting was added. Players can script weapons, vehicles, entities, NPCs, and much more using Lua. Scripted mods even work in multiplayer sessions.

To get started with the Garry's Mod Lua API, check out the official tutorials available online at https://wiki.garrysmod.com/page/Beginner_Tutorial_Intro.