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.
Kamilion, ELDragonne, ELDragonne, benjamin.rodriguez.lecaros, JasonVoorhees, 789tomi789™, Forest The Wolfy, itsmakeshift14, manoelmessias.j.santos, NikolaEdd, [RP901]_Randy_Sparks, Samwisehurley, kiralyszabolcs10, sami1925, TheBigZ, λƘsΣl3D, Drake3423, Bolt, DaveGames45, eDp! Scumb4g, kevoseven11, SickMan, beedyCZ, gamerisraw, jeffthekiller96, Beta CHEESE, sinpernene, comanche93, ASTRO_119, zenderdjmaik, IchaTV, Faliman, Faliman, bloxgate, Davchoo, Anthony, RobotJengaJoe, Jurisdiction, Murder Is Pink, John Mason, joakim_06, Tuk1na, Cpt. Rogers, irv.1992, sithlord1, xXTurkyRex04Xx(sadJuly29), neomew, Ruethy, Leep, Steinman +1 others