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.
kevin200224, Haydo0467, Cartohalive1, = FL =ViRuS, MRdoritos, [AJCII] COLORMATIX, FaddaFingLing, ZONT ADI, SiSO, NeLiME, ekimmillion123, Black Inktoling, LordOfTheFlies, Englandball, dmitriy422, TheBaconator, [ZHRP] [Legion] [uoᴉbǝ˥], Tachibana Depta, Saint Jimmy, Crooked K3ANO, Melting Terminal V2, Euroshaman, [T.E.G]EnderDragon350, =GEN= VAMP, Masley, Jesuka sumpson pls, derr, kimberlymyrick64, Mop, WaifuFactory, WaifuFactory, wangshiping3507, wangshiping3507, JoshDBoss, JoshDBoss, JoshDBoss, BLΛZΣ, DooMguy, Taradosaurio Rex, Riftraff, iceSpice, Captain 3Tears, OvahTheya, outgoing4325, Sam E. Dingle, Warpath-the-drone swe, [NIИ]Complication, hunselbahn, Dee, Pope Gutless Bastard I +7 others