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.
grimravus, Littlefoot (Bday in 2 days), SquidMaster, Windows, pedroelias0712, FIRE GOD, Cute_Bonnie1987, vovanbl64, FuriousRedstoner, bertisis, DPlusPlus™, Vandarxx, [FL:RP] Darkside, olivierchausse, olivierchausse, Marisa Kirisame (Cc), Dune Jumper