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.
felipe.gaioski, ネプチューン - Neptune, A Final Ending, NEINTY NEIN Cr0ck3ts, Pikacho, tpeasetiger, SurgeKoli, Temen, BB Stardust, CouNTdIKaFANG, Bloddybone, Quantum13, 우빙(U_bing), 안몽, ROVE, Sweepyoface, TheCoolKid, Grozger Zasrogtiveslin, DimitriPilot3, Toxic, duduklusener, ∩(≡＾ω＾≡)∩, *festive* Sucking Suck Of Suck, Redcuber, Willtree8, 빨감!!, FIN Kenny, Шпага_228, [KOR]joohan03, Batman, Pavel143, Vilsol, chevchenko99, 범인b, 범인b, DatAsstronaut, NeurofunkPunk, [Brony]monso, Deathloop, pizizup (check out my workshop), SIGMA, VANTA, Trainman_1711, VL4DST3R, Slash, Chin Chin, mavman88, Hamrick, CaptPatrick01, Draftyspirit +26 others