Add Mod Content to Hammer

For this example, a mod called "Portal Mod Test" (Game name: "mod_test") will be used. This works on all engines.
Portal 2 has the following mods included:
portal2
portal2_dlc1
portal2_dlc2
update
When you start hammer, it loads the content from the mod described in Portal 2/bin/GameConfig.txt
Portal 2's GameConfig.txt
"Configs" {
	"Games" {
		"Portal 2" {
			"GameDir"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\portal2"
			"Hammer" {
				"GameData0"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\bin\portal2.fgd"
				"TextureFormat"		"5"
				"MapFormat"		"4"
				"DefaultTextureScale"		"0.250000"
				"DefaultLightmapScale"		"16"
				"GameExe"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\portal2.exe"
				"DefaultSolidEntity"		"func_detail"
				"DefaultPointEntity"		"prop_static"
				"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\Portal 2"
				"MapDir"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\sdk_content\maps"
				"BSPDir"		"C:\Program Files (x86)\Steam\steamapps\sourcemods\Portal Mod Test\maps"
				"PrefabDir"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\bin\Prefabs"
				"CordonTexture"		"tools\toolsskybox"
				"MaterialExcludeCount"		"0"
			}
		}
	}
	"SDKVersion"		"4"
}
We only care about the "GameDir". As you can see, the mod portal2 get specified. If you now just copy the line and replace one of them with your mod's location, it will work, but you will always get an error message when you start hammer!
This is how I got around this:
First, we need to add a new mod to Portal 2.
Portal 2 will now have these mods:
mod_test
portal2
portal2_dlc1
portal2_dlc2
update
To create the mod, just create a new folder ....Portal 2/mod_test and create a new gameinfo.txt
Content of our gameinfo.txt: (Copy pasted from Portal 2)
"GameInfo"
{
	game 		"PORTAL 2"
	title 		"PORTAL 2"
	GameData	"portal2.fgd"
	gamelogo 	1
	SupportsDX8 0
	SupportsXbox360 1
	FileSystem
	{
		SteamAppId				620
		ToolsAppId				211
		
		SearchPaths
		{
 // When the mod mod_test is mounted, all other portal 2 game content is loaded as well.
			Game				"C:\Program Files (x86)\Steam\steamapps\sourcemods\Portal Mod Test" // <== The location of the mod
			Game				portal2_dlc2
			Game				portal2_dlc1
			Game				portal2
			Game				update
			platform			platform
		}
	}
}
What this does, is that first Portal 2's content gets loaded, and then our mod's.
Now, we need to tell hammer to load our mod mod_test instead of portal2.
GameConfig.txt in ....Portal 2/bin/GameConfig.txt
"Configs"
{
	"Games"
	{
		"Portal 2"
		{ // The GameDir is the content hammer will load.
			"GameDir"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\mod_test"
			"Hammer"
			{
				"GameData0"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\bin\portal2.fgd"
				"TextureFormat"		"5"
				"MapFormat"		"4"
				"DefaultTextureScale"		"0.250000"
				"DefaultLightmapScale"		"16"
				"GameExe"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\portal2.exe"
				"DefaultSolidEntity"		"func_detail"
				"DefaultPointEntity"		"prop_static"
				"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\Portal 2"
				"MapDir"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\sdk_content\maps"
				"BSPDir"		"C:\Program Files (x86)\Steam\steamapps\sourcemods\Portal Mod Test\maps"
				"PrefabDir"		"C:\Program Files (x86)\Steam\steamapps\common\Portal 2\bin\Prefabs"
				"CordonTexture"		"tools\toolsskybox"
				"MaterialExcludeCount"		"0"
			}
		}
	}
	"SDKVersion"		"4"
}
We're done! We should now be able to open hammer and see all of our custom content. In case you override any assets, like textures or models, you will also see those instead of Portal 2's.
Troubleshooting
In case that your GameConfig.txt gets resetted, just make it read-only.
Category:Tutorials