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.
Itachi [Didds0n], СВАГА БИЧ, Crestata, ShinDoだ.き.ら.いぇ, JaysonAlpha28, CSIRKELÓ, T.W.N.F.C.A.A.O., haloreachfolife, Roofroller, jabm0612, zombrex70, Blyasin, qandasunday, agustin, Jacks Teller (fondateur lnm), creedenderninja, jhonygrone.cuaji, Sold you for RP, ๖, ericeric06, Flamer, Igor00X, IrucxTV, joakim_06, Doomootron, Faded