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.
triblion2000, RetoE, DZG™ - CC | Djsimone #H64, Macra99, marzis, LewE, ☻ FerrKA ☺, KiriGuy, TheSniper, mikkel, Depax, *Wick*, My4You #BenQ*Falck.dk, Captain Whiff #4evarafk, Zimmermann, Ninja, TEA, SavageRP SachaTb, Dmac, The True Ryan Evans, [AP] Satoz, Han Solo dies killed by his son, SlenderGamplays, Kibito8, 000000000, thebrit05, [IG] Samyboyy, ohflow, Kimdoldol2, Freesun, hewnic, [LP] Monkeys - Join me!, [ L ] HorA, GioGimic, Homeless, Wepnet, [TA] cheesemarathon, Amiz69, KITKAT, ClaytonM456, ☭BlackShark☭, game54007, josh2233635, Zaku [Black], Panty Hose, Lebofly, Jona( ͡° ͜ ʖ ͡ ͡°), xlbvip, Aimshock︻デ═一, John_The_Lazy +33 others