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.
FiL0S0V, hakui_17aya, nathan126, call of duty master!, ykuear, ykuear, TREESHAUN, youcountre, Datadagger, SickGamingTV, kai_219, , KonradOPS, BOOM!, ぶい, yurikool_26, [Rainbow](•_•)Raz Yafe, ExeptTV, Mario44422 ヽ(⩾Ѡ⩽)ﾉ, ! ムcti, Wizard, marty_kit, Kyozhou, Radray Rotten, ph33x, Tex the Tepig, Tex the Tepig, thegypsykng, Andrjuha, Taken, abolkkj, Matt, Dave, єℓɑиɨs, yeowjoon99, mTw, DotEXE, cdaicuong2210, Sokkah Abdelkader, rscoywolf