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.
Gordi, Корней ☭, Dingasm, Yuuki, Pugmatt | Duel.TF, monster2743_YT [MGg2743 OWNER], Polux, NicolasSieg, Kiritokun, drenahmeti22, Mr. Sir Cowz, ᠌᠌᠌᠌᠌᠌᠌᠌, ᠌᠌᠌᠌᠌᠌᠌᠌, emanuelalexander1990, NiceGuy, PikaChris64, [560RP] dickson, Symail, obantr Sonsuz Aşk Mutluluk, obantr Sonsuz Aşk Mutluluk, ᠌᠌, Artes, Horizon, Mka0207, ⌠ℝℴℳᎯ₦⌡, MULEKIN!, DEADMONSTOR, RadicalReplay, Hoff, Yew™, HkSymps, Datamats, Starvox