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.
re15revisited, orcclans, BayPig (lego cunt), LeePyro, Lord TL [Hun] ☭, Spooky Scary Skeletank , Michooo, Warpath-the-drone swe, Warpath-the-drone swe, tonythezombiehunter, Jackie Saint, uberchain, PeculiarPsychopath