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.
rya.np123, Moltag12, Therandomgamer121, ariano113, Nuclear_fontra, E7ajamy, E7ajamy, [InoMaziaRP] Dylan | C |, Frazaer, Vexel, [Label XTrem RP] Yepco, Mr.FoX, joeman04, qocjd10077, some unfunny skeleton, GOMFSFB (MIC BROKEN), Caucasian Gigolo, l.a.helvin, Niko 98rus, Rodger; Feel the Bern, God Empress Sunbutt [SION], HalfSquare, BeatzKilla, =PP= 419, 김대, Mr.xXx (Sarkastik Demon), [MG] AlertIowa1234, Henke, Empty Password, Neon, Starvox, L3GIT KILLER