Just throw the BSP file included into the "C:\Program Files (x86)\Steam\SteamApps\common\GarrysMod\garrysmod\maps" folder.

VERSION NOTES :
	v1 : original (not included in this pack)
	v2 : original + easter eggs
	v2a : v2 with more spectacular ladder collapse
	v2b : v2a without moveable props in the secret location