Garry's Mod is a sandbox game built for Valve's Source Engine. It combines assets from multiple Source releases and allows players free reign over the physics, NPCs, vehicles, weapons, objects, and enemies. Players can build upon maps from games like Left 4 Dead, Portal, and Half-Life 2 while spawning in any of the elements from those games at will. If the player wants to just go crazy in an endless swarm of Headcrab Zombies, set in Aperture Labs, they can.