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.
ethanyan, C❤nstλChymic, 4p0k, 4p0k, 炫炫oao, fazafreeman, MrMiller, St. GIGA | trade.tf | scrap.tf, robi20001, Seaal™, Greta<3(no sound), gta-ozketgs, Captain Arashiko Tammy, Roberto Emilio Carlito, jQueary, CCNetωørk®