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.
Mr.Spinkes, scheuermilch_97, Lemonz, Jeremy, Quattro, Tramok, The Forsaken Templar, vittorio.cadei, Leo Nivorous, [HSH] Skyp1lot, Mori Krober, ElliottKing, bortonator, demond_xenomorph, romantyczka64, tiniboe, armando300apk, Ikoo, roboticcyborg2, Ash, berryarg, GenericGames, Migos, petertrifonovalexeevich311, el Trollo, MCrackLP, Naemy, Naemy, Naemy, puredominanz, Denise, Unding, Sliver, annikai, GAmeZStaR28, theCookie | Jan, ItsRobinLp, Cartner, Cartner, Swaggy, Baron Ordos, Ultivitamiv, Clownfiesta, Clownfiesta, MrNosi, Masuro, willianlsobogado, TheKnightRider, Momo_HP, Nytzi +392 others