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.
Swabra, Zerse, fatboob3982, Saman.exe, sandav22051999, Mr.OldschoolGamer5000, Ŧ㊉P㊉RIҞ, armansquare29, [Ethernium.net] LucKy, stasgubin.notcom, petersol2005, marvellous, Legion, skatezinco, MoLeX, ilad6508, KoShAk™, WAIVER666, Снеговик, Injurus, themanhacc, Garry F, 321XP, caminugamer, Golwer, Quazzy, HarryMat, S.E.K.U.R.O., SAY MY NAME, Error710, nikinson, doktorspawner, steviegreen77, ImJxP, bradyrussell159, MeIsStuck, Vladik, abi.sofiane, BLINDÃO, jem9055, willjsteiner, Hummingbird, dronix06, TheFrenchFurrie, mr.sandbox, RamzeS, James, XitE | Jackson, JANIDASHERPRO84, FrostLord +129 others