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.
TryHardGamer115, rohansmith871, alexshulski, archicastor1, kawanssilva10, dostpold, JPSNGaming, falseprophet141, KAYASE, ✘➥IPryse➥✘, minhquandoa, ☃❆[SM]RazorNight❆☃, ★λDUM JΔN$ON★, lloydbus, Slammin', James, Blueberry, Ur-Didact, GueSSwho, Wash, Critical Error, Drenamow, [CB]3M1L10(swe), Zokrau, Picklebond, viandox