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.
xNexus, durpachudurp, Quin, $$MORÐ!$$, [SXB]_Alesha, Very Spooky Engineered, Mad Matters, l.maire c, Zaku II (laggy addon collector), Dawnrunner, Crazy ol' Cheeseburgah, TOMMYE109™, Brad Darkstorm, adimn8000, The Unlucky Medic, LazerSpy