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.
lenguyengiabaolop71, marcin.juz, 96Beryl, Squeeze, Black Mesa Hev scientest, MaelRadecs, G10, TOO-BEE!!, nabilyusof, Lick Mike Hunt, VCayres, Sula-san, Upraski, jimmunero, Magix, |SliGz|REPŦILE, My Butthole Hurts....., Zard Wing, Cremator, mcLAREN®, , , =PP= 419, JimTheCookie, Yukon, Yukon, ☢DoctorNephi☢, Rev, Rev, BMBlazeweedz