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.
mojsadavid, Hard., re15revisited, Adamsbro, ¤Ð∫¤ PetRock, Deputy Huck, Loewenpower, tlasky.ml, Adv, Alex, tonimontanito1566, Arsium, -Toroux, maxiputo, truman1974, 3ducks, Nurwhool, ☆♬Green Hat♬☆ಠ一ಠ, bflmn, enzaim3001, 1111111, UBer, SerbianWolf, Vivi~, LuCKy Strike Games, LuCKy Strike Games, IsAwTrIcKzZ, DesoDesu #SaUcE, jeffthekiller96, Hunter, green, Grozger Zasrogtiveslin, MaXeLast, colonelpeach1815, Jess, 'J'/\K|, ☜═㋡ ƊɑϮ ℒαɗ ㋡═☞, Today i`m SolEk, ¯\_(xay)_/¯, Legion, Swagpaws, House, Thalia McKenzie, RadioActive, gloowb, Tommmike, ★orkunmen★™, avidsonicfan1991, ShotgunHobo, Ross +18 others