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.
Laviathan6675, CRQ500-[ZERO], Xlo0, 0shy, 10700murph, fearlesskdog, m.sutisna123, lincolnspencer13, Pimpsterono, Eden, Opo25, Sonic The Hedgehog, rocky4133, errollees, Prostar, Pain, adib4550, MARIO44422, [s3]Macross13[s3], ❅LeT☢Ňgue❅, •_Warsaw_•, jakeboyharris, GotGot le pangolin, thexkey | poot.net, Dookin Foof Lord, aldemir, Dick Kickem [TDRR], Benshots, Jazzie, muscleman74, marvin-mack, budlwsn, feast.jack, GoldiePlays, GalacticHam, FlushaDevSystems™, Mr. Moostache, desupacito, Authentic Fluffy Ken The Third