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.
zerkzees, |L.S.T Gamecontroller|, snuskete liten gutt, =SWFT= Fries, ariano113, _Daysu_, Leo, The Great Cornholio, [FL:RP] FGxDman, Teschke, CatNip, XiloFox, ✪Sheep✪, [No Fear], L337 PL4Y3R | trade.tf, HandBookGames, , cahenyard, JimOnDλPC, Golden_Freddy257, DeeZ NuTze, Icewine Rose, crosstheacorn33, | AG | JohannesNeo, TheSzabiHUN