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.
Grollgor, cloudcreeper2103, Barry Allen, Nigga.exe, hongnhat76, nØØb, RIKY995, stllok, contact, SFAriel, [BH]//Lore//, John, Code-Red, =TintedGaming=, ⓀⒶⓄⓈ Salt, @Wortho27, Natuca21, Natuca21, Puddle Of Semen, chuckfancy47, Bacon, Kalamitous, Spinex123, Omnipotiontated, MadDog, Struct, 씨발것, Pew Pew, Pew Pew, Hypatech, [BA] Brucektrain (DEAF), Keshwoof, Keshwoof, SainT, $2$:7!28(, disseminate