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.
andresdeleonpaniagua1997, That By fley, Savagely Random, Flying_Spaghetti_Monster, Mike Ookami, sebastianestrelladiaz, Big D, dennyS, Gorbathechef, Fairy Squad Mother, -§m- Allah Gold, afterthestorm454, diane, Norsfer, Hideki, Acrid appreciation station, Dalesome, 美しいです, Beta, ArkAJBoy01, 00miah, ♫ Wub Master J ♫, ☾Moonman221998☽, [Not Retarded], cryptidhuman, ScorpioCurse, falseprophet141, falseprophet141, fudiradij, Omni, Aids, Spunky Mei, MikKo, fursuit_sashimi, spartanluke, sygjh, glennjayxiong, 25082000ilia, 25082000ilia, 1246368102, Bee Babe, Asverical - V.I.D, Wizboz, judge judy, judge judy, xbox360home, decadent_uk100, OneOfTheZombies, JMax, Your Best Friend! +100 others