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.
Bowser2156, adrianbuyao175, sabinarohani66, Smokey , Finnwick FaggotBottom, Despite it all, LucidEnigma, Tobycat7, mrblue92156, incredibleDommyツ, AdaG, lolherceg8, mum, Squid406, jmario63, pizderman, ChainsawLord, Fire5kull, Yeayo, Jackal12, TheBoyThePlay, ★Roony205™, INSIDE, IverHR, felipe.gaioski, Entity303, razdolbai, mattpeckron98, 0, ZarenGaming72, ネプチューン - Neptune, A Final Ending, shotgun soldier, WhanTick, ArmyHD, tpeasetiger, SurgeKoli, Temen, BB Stardust, CouNTdIKaFANG, Bloddybone, omeigad, Quantum13, 우빙(U_bing), 안몽, ROVE, Sweepyoface, TheCoolKid, Grozger Zasrogtiveslin, DimitriPilot3, Toxic +67 others