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.
Plutonia1993, Creptex, 335 | ^4Haven, f0rele, EricClownBomb, mogimaad08, Gaylord The Autismo, 去他妈的你自己, Nfrommurderdronesgaming, sjroberts2, Stormie, Dynomium, Dynomium, Logan.exe, Albusdumbuldor, Gabetheman, Artem31019, Lunae, GAMMA, The_Dragon420, heckswagsmen, DarkDragon, Snerd, Cheapgroot, Woozie, jem bułke i sie ciesze, lourdzeanvalino, Never Nothing, AYMAN_cdDesk, А Калтофка Есть?, katanongmn, 2EZ4JC, Neondromeda, edikitop6, DeVibe - Searcher/, genanikolenko1946, Jakey08044, 4MO_C_ВХ, SdelanVCCCP, DinosaurXxX_DX, Отмычка, Horus, WanhedoSHF, tolf-dietram, tolf-dietram, AYKUT, t, ikaika_6655, miksc_473, ★바스★ +2225 others