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.
Fb||HeadHunter, Mr.Fps *Pt*, Ryo, evilgood2010, Notice Me Senpai !!, Alex (栗), CribDeath, The King Of Hearts, Fat rat, Nós não atender renúncia, bikkela, petersmail, Mr.Frank, FourTwenty, Guardian Angel, SupaDope47, Seathre, leo5652, SPACEViK1NG, truskiek1, Cable*, Toke, Muobman, Professor, Cornflower, BFL™ | BUILD | Remake, Bonsaift, Scorch, -ICE-, Keifer - Server's Back Online!, CraftDown, Dryestfool1945, Spade159, raygun gothic, |EBG| Brandino :D, Gambit, TheBananaThief