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.
rodonow, Noel, Policemanking / Hans Dieter, Esty, popcorne468, Макар, Scare Face | No Role Models, sparkz, pacino80, alessandro02c, harry 940, magagadzhief, PULSE, foryiizalt123, imboredbye, -V G- Gimic, applepielord333, John Doe, lcoyler00, hsgr, 1437481518, 「ZanFlowHD」, Rich Chigga, VCayres, V3ga5 #WHAT_I_DIDWRONGINMYLIFE, Dididreams, [F.R.T]ZinedineFR-YT, IvanFireX, AustrianNoob, Polski Stalker, billyviccars, RAMPAGE, milan_luca, leo65007, TheD4rkPlay3r, MegaVector101, Wolfgang #HASSO, Poogy, Rizzl #HASSO, some unfunny skeleton, truskiek1, Rayonaise, Dazzy, ROBahREEE, , , Dub Bozz Cat, Lord Eos, Munchala, AwfulMoney4341 +16 others