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.
Flying_Spaghetti_Monster, Matt, Doge, Adv, Dinosaur Rockets, lion12142, Tanman799, cubi290306, jc mek, I_Am_Sputnik, judge judy, judge judy, decadent_uk100, sreeharsh0, [GAG] MaDmaxwell, NovaStream, Sabun, [DPG]☣ ℳr.Crowley ☣, [DPG]☣ ℳr.Crowley ☣, lanebaldridge, tanatito, Wolverine Saint Jack John, LoneWanderer1895, Archer [HPMC], Mr. Happy Face, vigilante, Equinox™, appoXoppa, Dr. Snippy senpai, DexterSanchezProductions, Lord Eos, SKAYU, bibble, aor, aor, Luke The Damn Brit ©, Mr. Marooned, scumbo, XxDeathstarxX, MadmanMad, Foxtrot, The Insane Hunter, The Unlucky Medic, szikora.david, gas helmet, Addy