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.
DeadlyMoon7, Chip, Chip, Chip, mistertaters04, BallTh, Darkcraft8 [PhoeniX SPQR], Charlinks, Blacklaser555, -_Dark_Plasma_-, The Carmine, skywalker-10, jaybirdhunta, [FRA]™ Ducon [email protected] Joie :]]®, [SpieNat]Yogameister, yuvallll4, [Kna-K] Maximus Erectus, Arctic, , Sasker94, Linio, KING AARON, MrVideoFreak, The Guardian Of Light, Nightfall, dstroyer11, | HT.S | Comrade Bear, [G-Pro] F0x, [Syacorp] Syanure™, Salty Smile, [CSO]Refirser[모드 개발중], Terry Lost 1k ez wins, Thug'z, noi, Chewgum, Exho