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.
timzilla94, somaxs, Jeremy, [DPS] The Plague, TYNEZ, Shader1019, Nova, AGRESSOR, AGRESSOR, ๖ۣۜXenon, SpyCrab, EpsilonLombax, zwaaqwq123, [UNLA]MoMenT, predo.kozak, Deez | |\|uts, Aidoru Kotohiro, decadent_uk100, decadent_uk100, Steel-18, SrDirt, 후예, Darn, Horus, Fluffy Kaiju, Snipstein, Kans Owens, Yayep, Kersian, Solidus Graph, Ohmwolf, Striker8474, lDenixl, Deika, Deika, 3D, MadmanMad