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.
Fonzie, M4TTZXC, dotylake351 | kickback.com, pgthe14, BOTSlayer, TheMeta14, VexerJester, lucky014replay, xx31315, BountyHunter29, nguyenpenlnguyenpenl, xdino907x, Slaughter, ❤SnowBall❤, Bunseth (Still a virgin), ichillcz, vincentencila1234, RobosergTV, [Ciotact|operate], Just Another Otaku, Digital_Me, radik_melnikov, domlovezte, Okami Ken, lackpup, , Voxus, Syluxridley, interception, Lycanthropy, Soviet Union (USSR) (RF), Mr. Bulldozer [Engi Fam], Çağlar, baguette, Blak Cake, nesamone231, Blue Inkling, Whiplasher, Justsomeguy, purifiedHD, mn.guy, 卡卡超卡住, xxjdxx, werepig (cz), James The Libretarian, satio95, Error 404, Bruno The Otter, xleonq123, ballb1051 +161 others