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.
timmy999timmy, Durazic, Sr Doge ( ͠° ͟ʖ ͡°), A Special Kind Of Friend, pinkie, Cloven, Expl0dingLeeks, [ BRNY] DJ Derp3, DarkHermit, MoonButt, MoonButt, Skrylisix, Thomas, Icewine Rose, Drakus93, RoyalGuyRUS, justin