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.
eduardo.almeida22, Sashikko, ♠ ♣ Harley Quinn ♠ ♣, .Soυzα|тωιχχ™, chrysler.email, TheWeeyKz69, bael, 100913 | AnDo, andreitagsip, Haffiness, UncleSHREDDER, FiB. Dean, I SUCK DEAD MEN, jakbu, rifky12_naufal