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.
rmn1981, agent_gun, kosanovicveljko, Mr.Re, killerprohenry123, Sevler, colbypopkie, Driven, richerylsullivan, G-MON, fekete.kristof95, METAL HEAD, VitalityMod, achrafrebbaj, MaxReiger, 레포, [EMPG™] k0rAy1o01™, kpv8, Steely Dan, Helmet Doce, îÑ§áñêÇℛⒶℤÿ₁₀, Grigs', Admiral General Dictator, ILoveMoto, vikraal, (TheOfficial) Daithi De Nogla, Willymont, Rev