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.
ZONT ADI, SiSO, NeLiME, Ekim Million, Black Inktoling, LordOfTheFlies, dmitriy422, TheBaconator, [Legion] [uoᴉƃǝ˥] BASEDSTICKMAN, Saint Jimmy, Crooked K3ANO, Melting Terminal V2, Euroshaman, [T.E.G]EnderDragon350, =GEN= VAMP, Masley, Jesuka sumpson pls, derr, kimberlymyrick64, Mop, WaifuCake, wangshiping3507, wangshiping3507, JoshDBoss, JoshDBoss, JoshDBoss, BLΛZΣ, DooMguy, Taradosaurio Rex, Riftraff, iceSpice, Captain Kurtis, OvahTheya, outgoing4325, Sam E. Dingle, Warpath-the-drone swe, [NIИ]Complication, andreas.limsakul, Dee, Alcoholic Grandpa Rick Sanchez, DustyDan95, SkyCaptain, Mixelg, stachu159, THEWATCHER, Tomiņš