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.
Sunrider_Vanguard, BFMSAND, Avesome Vourden, fdg, novoselov_2006, amylo.1125, JJuanjo55, essalexmercer, cjh43088, falseprophet141, totaldrama2888, noctis.caelum23, hilgst1, Kuro Naichi, Chechen-Ru-95, Kenzo, Walrus18, Atlas Flame, Fistaschka, jeffthekiller96, Georg Friedrich Händel, Gygize, turkufiliz, playerofgames24, LoopY, Lord TL [Hun] ☭, SounDai, jdhr24, "Pop the hood.", spaceultimatefighter, A Deep Commitment To Konata, gerod.ford, viandox, Godfather_Corleone