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.
nyugi, www.jak1213146, Scrimmy Crungus, G10, tothandraas03, XXXReeceTheHeisterXXX, |TrIggA|, |TrIggA|, tuhegystudio, john_f_kennedy3, fabiff1997, NickyHayden, surfirecat625, sukoisu47, xxalex338, Varen, Black Jesus, sℓαүεявεαsт12, Sofiane-77, Cyber Ninja ✠, Fenix Legion, sull, lonely19682005, fabif1997, Cultist_kun, Jakemreed01