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.
joosep16, CreepyPlayz_, BlaxiYT, Reitsme, ColdHardDeath, Little joe, Hades_kl, kmati26, fnafboy10, Eu Sou Troll, TheRoboticSkeleton, thelogangamer, misasauce, mp5wizard, Toaster, SorrowGuy4, sexystormtrooper1337, Duffy Reis, re15revisited, blaisedyc, OmgItsDangerous, f1nn14n, RedXThunder, joao.antonio.franca.2005, 10081971z, SsMaxi12, 1337, Nico3D.de, Choripan Rushaholic, Rhinocharge, [OC]James1994, roxhacker371, TheDarkSlave, iamepic1203, Issei Hyoudou, jjreesthompson, VirusJ, Dj, Fridom, Didibeng, BananaBathtubsFTW, yoyoamr2023, Fun22, sss, LiessDu93, fourthwallflyer, silvershadow241, Engi's magicbox, Miss Pauling, XxFancyFlamezxX +329 others