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.
Mrs.PiggY, bence, Matesz:D, Arktos, ThatDerpGamer, NeonRage, re15revisited, ☻-Quakes-☻, king_pokerface, [Rizen]drag0nhunter1, abtekk, Johnanater, Pro, MoonChickenDogRat, Dankest Memes, Hello, its me, Team Static | Smiley, ๖ۣۜ Bob ๖ۣۜ, diezoR < FOX >, Kooshi, padillaangel645, kymani0221, KingOfTheNoobs, Trizz, Henytir, ArkinoxGaming, [DSTM]☣☢Bryantdl7☣☢, |Konamichs|, Gem, Ethan, G-MON, Voltan, gavriiuc.lilian, Nimimerkki, DustAmac, DeadlyWoods, HyperGodz, Decystical, -GG- Lincoln Clay, Gerry S, Connor Harrison, VladR, Ghost Prodigy, DaddyDrunken, . :Engineer BOY: ., Arthur Crannel, Bhhenry, Winter, Ƥυℜρƪ∈χ'đ ƊατǤυ, The Doctor +3 others