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.
Fallen.>, Sergeant Squirrel, PC-Chopat, RexxlxD55, RexxlxD55, BenjiFavoXD, Dr_TGH, DeathArrow_xD, K-Popper Ql, cuck, kingstone, [LBRP] Jamel Warston, [LBRP] Jamel Warston, Kobster, Yung Walkie, ezekiller062 |FR|, JE SUIS FAT, phaze, phaze, Lucifer Morningstar, Neon Smecta, CatisM, Psychotic Decay, Lewd~, gamerking2051, hanshinkyojin1997, Wtf just happened, Khan, b0j, DemiGod10115, Ki'Yuki, Andrès De Fonollosa, củ_cải, Will Tait, PineTr33, Iwau, Flui hellcase.com, wza_Mentix, ^6Ramen, TheDeplorableGamer, avogt210, [NSG] Jack [Founder], pedrogam615, Nathalaia, lahirueranga11, zadquielortega, Jokar, Front7HD, DIDIER, The Great Harry Hoodini +320 others