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.
rmn1981, qaqso12, adujsh, ekremsuhageyik, joselitiko1975, ツBamibal_Des_DOODSツ, jgvg24, islemDZ, AbdouDz, Sushie972, Lautrec of Carim, JaarboekH, Steely Dan, マテウシュ, Aneykov01 [METALHEAD], Azrod, dimicharamis, junjun, funkyblackcat17, EthicalPerson, salaatr, George MARTIMEL [FɌ][UJP][ɌP], cfendt924, Supraline, www.couraest, People all grow up to die, Leon04