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.
distantsister, ComicRya, ™, lilitopito, reyesacosta21134, Zekhiroy, tristan420, Angel Aguilar EPN, konic2003, xXolpe, pabloarraiza123, Soup Bowl, Tails19935, ThatNintendoGuy, snipexariel, aubytaylor, swagpiratefox, KEL_Na, roylereg, K341912, Takaruichi, AngelJasiel, Xeonas, ellococharon396, zackfancher13, Benitoad, gamercouple81, get urself an outfit, then64man2, s.nachev, Bεят54, axelnievas22, MarioKnightFire, lucadellanna0, lucadellanna0, Mar9202, DR.BEN, joltive, kmanblow, KalielBM, Commander Abeloth, Erazik, luckyboy777hahaha, jnoski92, seluakn, napalmmc11, GamingGamer, TheGamerPlay, Braxton, Fordon Greeman +73 others