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.
<3 ESRAAM <3, iLogix'., zeky, Liquido, Uvuvwevwevwe Onyetenyevwe Ossas, Enixpho, Radical '80s Black Man, Hoixi, AGGSiH, re15revisited, Wahed_KongWengWeng, ็, 123seven3, twofacetoo, Nobly -BOTJAUM-, mL1802^^, SpunKie1337, MudoFalador, tizubogdan, Salmon-Chan, kundavid041555, [FANSNYARAISA]+++, Foxy The Pirate NV, Mister Strange, Cam2000inator, orbital, The Sexy One, LO.W, HUWAEI :", Herp, A Blockhead, pierrethebest, eaEskici, Rayz, toutseul16, Pdive, Guiyai, tom.jones4, Robot Cat, Anonymouse, Wiggle Playz #pinecone "skill", Gerardnatious, ɛʀʀօʀ™, [email protected], Mr.Magnificent, TheNextGenius, carinho, Jess3D3, Griffis520, zZzKillerzomzZz +72 others