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.
Collin Felde, $assy, davifilipetto123, GhostyFace, KenoN, Coma, Hason, Binary Space | SWFS, Binary Space | SWFS, Binary Space | SWFS, GeeMayn, XDbomb, KarNal, Jean Petit, Левое Яйцо Кабзона, Ŧȿϟdoctor.y, Worm, [KG] Muse, Eskil, Eskil, Refirser