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.
skinup.gg Tuki, hero.kutasiewicz, physiological.exsistence, ХyŁîĜắŇ4Ĩķ, nightrainmusic, Solermaxno, sekoudia6, Stevie47, phamthien921, gachi_bcn, The Medic from (Meet The Medic), mineircraftgamer, fazafreeman, djermoune1666, RazerBlade, Frostealer2000, jyrenjyrene22, DarkAssasiN, busermanov5, TheSasGaming15, luizoctavio56, ric2017 (G.E.W.P), jollyxD, Downs, The_WonDeRing_Boy, yasser, r-six, Finnegan1611, toughman52200, Lifebox, DiegoalexHD, elpablo01nick, msatour51, DaRkWoRlD BTC4KEYS⸳COM, orla