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.
ALCATRAZ6179, twitch.tv/messiasx7z, Capivara2023, Bipper5, DIMEBAG DARRELL Jr., DiHMé, 。゚( ゚☯ ☯)゚。, lautarovergara900, Tacticusal, black cat, maxirodriguez.95, ChoChoTheLhaLhik, papidefeu, jo20885uk, necat997, AloneDark, akile, Asta, GildMan, GildMan, peexerdoodle, Olesław, yaash2323, BenchmarkGamingGeek, 6icada, CasualVillainy, Batman, Sx-, amirazh960, xManu05, gordonpedro459, yonajgb, rick_hp100, [TopG]Malc, CaseOpening.com_Filip, seixo_papinto, HARUKI, gasmihamou626, Spartan, sajadmax, buellertech, 3djuegos2016, dethknot24, Srgabrielnotz, tatulamemey, TeknØ, Gabriel Costales, Mehcan123987, alexander.mafra, yaousuo +20358 others