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.
Shuttle-X, zackadiax247, Deadly151.#, gmamba73, ŇO LUCЌ', PlayerofGames, Meowskers, Crome, KoS, Kayman5000 (K-5), HIKINITE, tfoxn 42, bbatmanso5, Tails, Salty Toast, not annette, EternalDarkwolf zEGCz, anime boy ;), Mr. Lamborghini heavily Sick, King_Cripple, Tim, Chase Hall, bullshit, ☢ Krauss ☣ Boosto! ☢, Ratchet, Ratchet, [BA] Brucektrain (DEAF NO SOUND), vokster117, DrGatsby, John Cena, John Cena, John Cena