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.
hexer54539, Hallamaa, _Anime_Ken_, ..:::deceitprosuorhan:::..., xoraz, xoraz, Revenant6x666, Samu, umutyildirim383, a "JOLLY" Male :3, izzy.email, unclebody, bolamanteca, desinteresado124, Thomas, Balkaptan, Xelibix, .jbug, Yoh, Hervy McDervy, ๖ۜ§tealthPaw , crapstik, CONRECON, KAPTAIN KRUNCH