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.
mossesx, Heckelex, green, nagymark696, Desolatora, Manu, Googooeyes13, proli2000, proli2000, jamesmajora2, spaceultimatefighter, ИЛАРИОН-ГЪЯРЯК, l.maire c, Pepelucomelka, Wolfman19682005, lambo18, Jimmy33650, [BA] Brucektrain (DEAF), Sonic Alchemy, TearsOfDream, CONRECON