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.
Swin.WoW, Embar-Go, BOT Wyvern, Little Crumb, firesip365, firesip365, na.nabil, rtgoins69, falseprophet141, rabbit, Down In Graggle Rock!, Commissar, GLGPRO, nasfat, nasfat, the.mad.nimrod, Huevolat, Zachariah, 레커, 레커, armandoalonso1, Stratos_Sfer, Stratos_Sfer, Vladimir Volkez, frienca, kjelli_, 왕을 쓰러뜨린 자 Noel, Kumdong2, єℓɑиɨs, YeahDoodCzLp, soundwave(dead), lonely19682005, antisocialone, Dreadmaster231 - [Offline], Dreadmaster231 - [Offline], LiquidFire, R U Ready?