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.
HawteX, tol.do, Beleti, blohin.alex2016, heidicoles563, ///Nyma///, Scotland, The Jackal, arm-?, newonhuu48, joawnai, Pingu, jeffthekiller96, QuinCraftables, djwildchild, _Appler_, noobdantds, wangshiping3507, latarrito, cesardavid.rv, joakim_06, ToxicBarrel, Spyem91, Jarcie, StreetZ, $∀Я!Ξl, plushyfilly06, yeowjoon99, axelito42, stevehughes1949, TEHDIAMONTRULL, plastovnik, Zokatar, thirsha4242, furdok72