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.
[FR]EytanMmn#YTB, Fazeur [1K], CrackerFR | Rewards.gg, tom4Hawk, lkfblackwhite1, MaxDu56 [FR], Shogun, MR.X, Fizz, Jason, Roy, PhantomKnight, Howzy, TheDmitry, multis_jeux_333, Sur. Sifi, MIKA, WarSnFez, [KOR]Green, Henry Lange, ¤ Roulio ¤, Gaming_JR, Ambient, justinee.serano, s00L, Ch3ak-, Flame, [NV] 50shadesof4chan, PE Networks Owner| Church, Julate[FR], Corentin49, igneouscheetah, NiFFive, [ROH] Kaito Kid, [EG] Zeo, Jean Bon, Volvo!, Украинец v2, [CG]harvjomac, Mr Patate, PixelzAzzort, Revoke, Rhiz, Shiina, _-Mr.Ubivashka-_#Get_Rekt, цвяточеГ, Soddin' TicTac, Xemith, Xemith, SwaGG227 +846 others