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.
masascuate666, Shredder, 约翰·普莱斯上尉-, opazofelipe95, Maxeloxx, AssassinsPerks, Player, GAZ, Soldier, re15revisited, RoboLV, Sandball, absza01, Russia Greatest Love Machine, SVRL_, swat0921, MrGeRoI, Lemony Link, airrice, Bora[TR], DumBoCow, Noob Saibot, LazerSpy, Dune Jumper