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.
RGs_PREDA, 秋海岸, 2brosplayminecraft1, Mizumi ★, McFabb, xXx_BagZ_xXx, boomix, kalfinio, •̪'●Myrrdin•, Nykez, Tahlian, staffordmichelle9, TitaniumSpyBorgBear, Alex, megabricola25, megabricola25, megabricola25, FRANCISZEK MAGELLAN, BLACKM3SA, RainbowGauntlet, ACG | Brand0n, ERROR, MicroxD, BLEND, CONRECON, Jason11040