This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
poompoomey, ElChamoSinJuegos TwT, lorezcos3ldon, wesleyfrancisco21232, CharmyTheCharizard, Bravo2YTpl, JoeMo42, GuinsooBoy, Misfire | Clara, Jacket Hotline Universe, BigBootyLuna, m.baghstany, arm-?, MapleMabari, qSenk.Gus, guyrot2010, Zlakovoe, Benio