Not all games distributed by Valve are compatible with Garry's Mod; only games that take advantage of Source's physics engine are supported. Here's a list (as of Garry's Mod 13): * Age of Chivalry (Free on Steam) * Alien Swarm (Free on Steam) * Blade Symphony * Counter-Strike: Source * Counter Strike: Global Offensive * D.I.P.R.I.P. Warm Up (Free on Steam) * Day of Defeat: Source * Dino D-Day * Dystopia (Free on Steam) * Half-Life 2 (Half-Life: Source, Half-Life 2: Deathmatch, Half-Life Deathmatch: Source, Half-Life 2: Lost Coast, Half-Life 2: Episode One, Half-Life 2: Episode Two are also supported) * Insurgency (Source Mod) * Left 4 Dead 1 & 2 * Nuclear Dawn * Pirates, Vikings, and Knights II (Free on Steam) * Portal 1 & 2 * T