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.
Epicguy5000, [FFS] bho, diegotomeldan31, TomaToe, CУЯ33✘ツ 「Hoide」 Honcho, Cabbage ◕ ‿ ◕, ✪ Satan's A Watchin, bence, Matesz:D, Arktos, ThatDerpGamer, Neon, re15revisited, Waffles (AtomicRP), king_pokerface, [Rizen]drag0nhunter1, abtekk, Johnanater, Deinati, moonchickendograt, ☭️Soviet Exo-Skeleton☭, Hello, its me, Team Static | Smiley, ๖ۣۜ Bob ๖ۣۜ, diezoR < FOX >, rawr c:, padillaangel645, kymani0221, Scotty Sutherland, KingOfTheNoobs, Trizz, Sofyan. QuinnRP, ArkinoxGaming, Akira, [DSTM]☣☢Bryantdl7☣☢, |Konamichs|, Gem, Ethan, G-MON, Untek, gavriiuc.lilian, Nimimerkki, DustAmac, DeadlyWoods, NovaStream, Morty Smith, Jophes, EPIC_BrasX, Gerry S, Connor Harrison +13 others