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.
Sans | trade.tf", re15revisited, H4Z3RDT4SK, Lucy"Melody"Addams, Brian M., Syd - Kammachon, West Coast Psycho, ☭ Lord TL ☭, wired, poketwinsblackandwhite, -=[UH]=- General HeX ♥, Admiral General Dictator, Cap'n Dickbeard, Warpath-the-drone swe, Warpath-the-drone swe, lonely19682005, peytony9, peytony9, NotWaitMe675, PeculiarPsychopath