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.
๖ۣۜMoosicorn, ๖ۣۜMoosicorn, PixeL, Jesus1g, Maamsou.93 , i3G | Dev, tobiasxz, Cirilo Negao, Sgt. Black tikle Mh Balls, [ち:き] ❤Miu-chan ロリ好, PoStMaN, MrRoy, =TDC= vicepoint5 [stofzuger], Mr.Robot, Svensis, stev, PonyEB, Taken, WhoAmI, Steven, JackoLEE! or Lematou!, Garry, Xelo, Metal Fish, Olek1223, |Z| Max, |Z| Max, spid, DeStilleGast, El Chapo, Perky, PB|Dedruss, dragon99z