Copied and pasted from Interlopers, all credit to Kosire:
1. Open Alien Swarm SDK -> Edit Game Configuration -> Add
Edit so it matches your CSGO directory:
Name: Counter-Strike: Global Offensive
Directory: C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo
2. OK -> OK -> Go to C:\Program Files (x86)\Steam\steamapps\common\alien swarm\bin -> Open GameConfig.txt
Edit so it matches your Steam directory and get this in:
"Counter-Strike: Global Offensive"
{
"GameDir" "c:\program files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo"
"Hammer"
{
"GameData0" "c:\program files (x86)\steam\steamapps\kosire\sourcesdk\bin\source2009\bin\base.fgd"
"GameData1" "c:\program files (x86)\steam\steamapps\common\Counter-Strike Global Offensive\sdk_content\csgo.fgd"
"TextureFormat" "5"
"MapFormat" "4"
"DefaultTextureScale" "0.250000"
"DefaultLightmapScale" "16"
"GameExe" "c:\program files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo.exe"
"DefaultSolidEntity" "func_detail"
"DefaultPointEntity" "info_player_terrorist"
"BSP" "c:\program files (x86)\Steam\steamapps\common\portal 2\bin\vbsp.exe"
"Vis" "c:\program files (x86)\Steam\steamapps\common\portal 2\bin\vvis.exe"
"Light" "c:\program files (x86)\Steam\steamapps\common\portal 2\bin\vrad.exe"
"GameExeDir" "c:\program files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive"
"MapDir" "c:\program files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\sdk_content\mapsrc"
"BSPDir" "c:\program files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo\maps"
"PrefabDir" "c:\program files (x86)\steam\steamapps\common\alien swarm\bin\Prefabs"
"CordonTexture" "tools\toolsskybox"
"MaterialExcludeCount" "0"
}
}
and you can download my own FGD I made for CS:GO here:
http://kosire.dk/csgo.fgd
(Includes spawn points, env_cascade_light, buyzone, bomb/hostage, weapons, etc.)
It worked like a charm for me, but a few people have had problems with lighting and a few entities.