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.
Keith Keiser, DarckShow, [MSK]SkyLu, Eaa619, LCZ | Riptor, lobost333, gca54299, Your_Lovell, kacper-wieczorek2006, 7hegamerios, jesusdgarcia, xSalt_n_Pepperx - TWITCH, JoshShadow123, Pablo_Rx, connorrife1, grndthefauto12334, SUS CAC, madonandrei, Steps, thatpickle, Nek005, skrzypol678, leebritton99, tompakevin, Thaxtersmith14, Mark_c14, C#, Smore, KANO, Master Chief, yamahonkawazuki, yw, egekaraca05, VidioCrush [Reputation], Big Razza, imscoobywiththedooby, ShinDoだ.き.ら.いぇ, c.yassine.ronal, KazulBenkaichi45, Bmxbandit23, jond0000, tryhard'man xd, ♛MasterLifeRp♛Kylian♛, MisakiYuya, MeguMean, drumjedi76, zadquielortega, CyberCitizenZ, Noel, marcelo.tica +536 others