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.
Sqt, SevenPandas, Nayioma, BW //, Chyphix, Dr.Cynen, chiara.mc7, sLedz, JustAl3x, Credox, AnoHito, [GER] Lux, NiFFive, DooBiiE, JeezLouiseLemonMcSqueeze, Chanu Menobly, "Kjaerbye", Taqza, Deathtitan77, Bernard Tartare || theop06, Paralyzed, pippengerrobertdalton, rampart, Mechatronik, [KOR]혼살InLv, Roxa the Neko, Razyel, TerriBad, Yho, Natolikin+, getyadogsoffme, kendoggy, RainbowGauntlet, dirtpeanut, MadGentlebruh, Verdict | GarnetGaming.net, eyyyad.ac21, O'Nesix, Countsmegula, Dune Jumper