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.
FemPyro❤, yamahonkawazuki, AnthonyHD, kmahadew8, aminmeh123, dsadasdasdasd, vip.narancs, fightherot, pewdiepie, erictolley94, davideswag03, gorditopanda005, xablauvoador4oliveira, mj01032003.mj, karl1514, darbecc31, masterlagartox, Piter La Anguila, thawipoom, ikamuse johnson, Lightningray3, Pixcraftgamer, a1_ahmed20, Biahwtrooper, Ep 1996, BlueBlaster CSGOReaper.com, Hellasius, joakim_06, thegoldensteel, Sparkle, MasterThief, SpoopyLegs