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.
jnccomeau, SPY 帥哥, [LMC]Doraen, rya.np123, Moltag, Therandomgamer121, ariano113, Maxouuuuu., Nuclear_Shmatt, E7ajamy, E7ajamy, Vall, El Señor Pollo '412', Wiggz, [InoMaziaRP] Dylan | C |, poatato man, Vexel, Yepco, Mr.FoX, CoT, joeman04, qocjd10077, Jurisdiction, GOMFSFB (MIC BROKEN), Creed, Caucasian Gigolo, l.a.helvin, Alex, Niko 98rus, Rodger; Feel the Bern, Rapadant Hosting|Vaas, HalfSquare, Hikka.exe, BeatzKilla, =ΡΡ= 419, 김대, MrGeekabit, Mr.xXx (Sarkastik Demon), DrodA, [MG] AlertIowa1234, Moon_Bear-CivilGamers.com-, Henke, Empty Password, Neon, Starvox, L3GIT KILLER