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.
TuwaTuwaTime, TroyTheCoolGuy, re15revisited, re15revisited, ƦᴀτcʜᴇD VipᴇR, (kor)baboman, neecirno, ☭LENIN☭, gabimagloviti, 0000___LUCIFER___0000, lhjun679, 플포, Revolution3301, Illumini, Biahwtrooper, Karl Hanke, kdragon410, 膜蛤不如跳舞, 膜蛤不如跳舞, oscalito, 울트라홍, __(‾◡◝ )> David-kun, SetCloud, Realidie, Miller-Desu, Miller-Desu, 물통, Tark, Tark, Warpath-the-drone swe, maruko, [Louat]Neom, Jackie Saint, Jackie Saint, David, TheWinComp, stabkamay, CubeOfIce, Eraser Head, ChairmanVitorHL☭, mikakater, 전빡빡