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.
s1ngle #cah4e3.info, GermanGamer, GermanGamer, GermanGamer, GermanGamer, razortrap460, Kenbi6, OVERTOY, brendan0620, filiusarthur, Blueking52, ProViruS, rodion_karnaushenko, fightherot, Jun, kbubuka, darkplaygames, re15revisited, ０９-１５-００, Avesome Vourden, Gallomimia, Slick Willy, zanejulien1, Synx, kurak, 1907068117, 97batu, 45.u.2016, nakkalokesh966, Omega, redscopegaming, ChalkSauce, falseprophet141, falseprophet141, falseprophet141, falseprophet141, awesomekid, ϟ ۣۜWolf Furry ϟ, bengelch20, Lemon Ice, futo999, XKore, omarfalja, BADASSKILL, decadent_uk100, ✪LiqvidPlay, JasonVoorhees, Ragingpanda, MR.LOS', lee403980 +70 others