This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Twynix11 [FR], x9nine, Navi, ChengYun, Josh, Dunkel, Sassy the Sasquatch, Hidey, Spud097, HelloWare, HelloWare, Go'Vic.Ϻέψ, Pingu , Pingu , Pingu , Root, Comrade Stinger, Gafy1rTW, Kenobi, Sitron, Crolab, aivaras.pupelis, Greasywookie, ๖ۣۜNathan, Pranong,  Dj_ames, Cenn, Father Jimbles, Rutabaga The Architect, tennisblaze360, sanderla, Perfect