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.
yohannsitterle234, seby_nike18, raphitos gaming, funagoshi, Urbanichka, NidroMan[FR], BountyHunter29, North™, o-kolpakov, Marco | Vladimir |, Yuriy, KylianFRFR| Codeur Gmod, desinteresado124, TitanMnsM, ryanfitz42, Dkonecny22, ALANATOR145, Hash Slinging Slasher, Simon, Yohan.A, Dotrawr, ★MvT Pyr0★, Fontaine, SurfixX, ZeK, Darkwolf