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.
NicolasRenanMachado, zskvor38, Sonny | The | Gamer, mandr2017, sam1995agent, XTRON[ESP], 2 Number 9's, Ramzee, lolxdspawner, [CG]ChAos, NobodyTheThird, beedyCZ, 514citron, letop06, Alarm, Chris Almere, mandyslaughter, (PDP)-ɱiguɛʆɱɑzɛttѳ, [UMU]Rewolution[PL], ZpIrIT_99, FiLzO, nev