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.
lodusdog, alexmelendo, RevealtheWolf, yolobro72, DauVuong, N0T..$0..PrO, Shanpy, sunjica5, theicephenix, sauddvds, diogo.tavares2, williamsliamedx0, NeverLockedOut, zbox, diakou37, TechProvider, faresmezenner, CiferGamer, carrowayjoshua, skyminerprime, RedOdrin, yamahonkawazuki, nikolasdimitris, TheCrazycakeman, matthewhoover247, StReLoK, jeanevenselien2000, Beto1807, toasty, daniel_xx_x3, hotshot9500, Jtkk33, ITeee, adriano_sgroi, DELETENES, Marlon, theodoreatson2, jang436, bucsailol, 10700murph, partymaker.forever, [KOR] DAEWOO, ZONT ADI, dance-toni, lucigokus, benplaysgames4, JustBoss, CheshireST13, StarSlot, SirZgnSama +203 others