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.
Ledare Adriano Polimeni, Liz, Cs_Office, Adrenalinium, Shuttle-X, zackadiax247, KiritoGGLoL, Deadly151.# Dev, gmamba73, ŇO LUCЌ', PlayerofGames, Meowskers, Crome, KoS, Kayman5000 (K-5), HIKINITE, tfoxn 42, bbatmanso5, Tails, Salty Toast, not annette, [GOH] Master Of Holland ✠TGM✠, EternalDarkwolf zEGCz, ThugLife, Sora Tatsuo, [Miku] Darthkingshow, King_Cripple, Shinji, Chase Hall, bullshit, ☢ Krauss ☣ Boosto! ☢, Ratchet, Ratchet, [BA] Brucektrain (DEAF NO SOUND), vokster117, DrGatsby, Thomas the Pilot, Thomas the Pilot, Thomas the Pilot