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.
Lord Nacho, rok50, jose-patricio_figueroa, [APC] G Froman, joea, jesusdavid_150904, jorjo5445, tonywan2000, oprichnik, The Vape Cracker, Mbah_N4k4l, loupgarou_e666, noel16, ..//.mr.//, vinivallim, steelwork, Hoppo Chan, The Insane Hunter, Uros Uki, golden freddy