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.
JuanmaHL, seriou*****, Mr Justin, professor moretzsohn, professor moretzsohn, Killer Memestar AH, Forb, Darthcheese2, Samy, Havora, Beta, Beta, Beta, [HE-Artist] Eric Prince, Selim Yaşar, Nightlyℋ, jason2010, olinux, olinux, JustPlayer, -=JokerR |CMBG|=-, Wareb