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.
re15revisited, JezzaRat, DragonFighterMR, Gentle Moa, [email protected], LMR_1, GeorgiePotatoFace, GallifreyanGirl, The kitty of your nightmares, holiday_undefined, NEW YEARS JOLLEH ASSFUCKIN JESUS, Warpath-the-drone swe, zombiller, LazerSpy