original question: what Video Game Engine Developer developed a Video Game Engine that is Used for computer games [Garry's Mod]?
answer: valve corporation
answer type: Video Game Engine Developer.
auxiliary information: Garry's Mod, is a sandbox physics game developed predominantly by Garry Newman and later Facepunch Studios. Garry's Mod was originally a modification for Valve's Half-Life 2, but was later made into a standalone game. It is available for Microsoft Windows, OS X and Linux.
What is the name of the Developer who developed the Video Game Engine that is used for  [Garry's Mod] computer games?