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.
jisung, Merlin, Paracetamol, ID-xi, Peheneger, Sailor, Swan Ronson, ULX_, Paluchini4, Cyril Figgis, triblion2000, RetoE, Dylann, Macra99, marzis, LewE, deqoosh, KiriGuy, Nman, TheSniper, mikkel, Depax, *Wick*, My4You #BenQ*Falck.dk, Captain Whiff #4evarafk, Zimmermann, Ninja, Vlad the Dragon, SavageRP SachaTb, Dmac, The True Ryan Evans, [AP] Satoz, Han Solo dies killed by his son, SlenderGamplays, Kibito8, 000000000, thebrit05, [IG] Samyboyy, ohflow, James, Ticklish, Kimdoldol2, Freesun, hewnic, Comrade Drew, [LP] Monkeys, 햄찌, 하늘, [ L ] HorA, GioGimic +49 others