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.
cbothma, lucanluca7, 3rm4c, TOT0886250470, heiko.klopfer, manbba123, Sam[TIA||RHC], lolepad.cry, KILLERLTUK, becebece_bece, Bomba, KOMUTANLOGAR74, tawseeftahmeed656, Kayxy, Xen_12_YT, jeffthekiller96, pollopera250, Umar2002