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.
prueba.gol, leafhefazli, spartan119990, ariano113, freddy.kacper, kiralyszabolcs10, Castor, faizaan1muneer, EnkiAnubis, =>[[email protected]]<=, (T.E.V) St4yFrOsTy, Scooter, dsrdakota, Kaboom36, LpSuchtie, Daniel, Mr.Jeicenas, assasin, crazyboutrocko, TheSzabiHUN, Takahashi (Cheesy), Xman2820