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, Seamus Mcgee, [email protected], [email protected], [email protected], LMR_1, GeorgiePotatoFace, GallifreyanGirl/JustALeafNinja, The Kitty of Your Nightmares, purplenull | Jadatness.com, NEW YEARS JOLLEH ASSFUCKIN JESUS, Warpath-the-drone swe, ßácõñ, zombiller, LazerSpy