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.
Humbleness, [NG]Stezzers, DedaVasy, danifeatnigguh, Mr_Piggy, Yamikai, Hoje, ÇrázÿBrõ, Jake T., シングル ウルフ, |Z|I|N|K|O|_Xereon_|Z|I|N|K|O|, fredddo690, l.maire c, Dy Sung, ☯≤<Nikita>≥㋛♛☃ ✔, Nidolai, sdp-gaming.com | SDPExtra-Large, KERRY, |QIG| Brian873, jOY=, Jack, MrKuBu, HARVEST, Pugs, ¤[FR]Citiizen¤