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, re15revisited, QTG坑, Sov, Boogaclaw, salo_S94, Legoben2, Jurisdiction, techmike117, [TW]SharkFur-Krahs, (1-UB) Bear Claw Chris Lapp, Shin-Gojira, TF2-Reddpie, Warpath-the-drone swe, PC1073, Minimole the Lorax, Jensen five, lonely19682005, Mr. Wilhelm., Mr. Wilhelm., I GOT BOXES FULL OF PEPE, Dis Dolphin Guy.., ruminaas96, LazerSpy, LazerSpy, SevaGlitch