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.
pooh112233, Santee, Gr4nN9Ba5h3r298, s13046, m.sutisna123, Synthersize, Swizh, Fabiola, SLEDGE_HAMMER, irenethour_15, baderlefort, Blerfeurr_gaming, pA|kamika.XAMA, iobere, furqanahmed146, gamer, sinaveyci, benauguere, wjodaod, Babico, Halowarren, tbadr21, mant_29doesnt.2005, thereistor, mamba69696969, captainrex0786, captainrex0786, MAD, ppr5092, jack17717, ryanator135, ikhwanmeow77, anarchy.2014.sb, VedaxYT, Pierrii, Star, josedavidcs02, rdonat123, sylwek-ret, luckyslevees, rafaelmartins150606, Leo, len7, bryan, Blackens, nazrulnasir4, Luisbarth79, frost8622, 794715, just a wolf +1053 others