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.
Duncle-Sans, SIGRITY, daniel_pagel, Uloldb, GaiaImpact aka JudenLeger, (SONIC), ✰ Šüpęr ēthīçâł™ ✰, Bossumeister, XDbomb, alwinboerman, semmiertelme14, quoctai101, JEFF THE KILLER, TheCasper, Resul230, $ervqL, Iansomnia, juanmejiaa3, Max, DeesCacho