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.
chicen95, ZosoMan, luis15cn, EMITEK, bubby, // звучи CTRL //, dei21319, Guilhack, tdbcat, re15revisited, re15revisited, SirGibson, nyu, guns_matt, Joe, Saysay, Herra.hj.lol, Ack Ack, rongta, Spectre, cjdoniego, MusBlaze, Sound_Extreme, The_koo, djshoun3, Nauoky, Nauoky, Cryms, file_follgen.mp3, Liam | WickerMan, sinius, Frost1551, Gamer17, kingdude84, Pancakes 4.0, thirsha4242, barukkamu, andrewdbuilder, Dobriy