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.
SirGibson, nyu, guns_matt, Joe, Saysay, Herra.hj.lol, rongta, Spectre, cjdoniego, MusBlaze, Sound_Extreme, The_koo, djshoun3, NASVAy, Cryms, file_follgen.mp3, Liam | NewNoize, sinius, Frost1551, Gamer17, kingdude84, thirsha4242, Barukkamu, andrewdbuilder, Dobriy