This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
General Pinochet, TioArchi, Leito.- ツ, Harvey, Uhhhh well, some guy, Kragosh, SlidGuzman, gaelgerardoramosdelgado, Maxi Dixon, zacy5000, [email protected], lucas56bkn, PoG Tibuzz, Swope™, EDJE, Lubricant Jam, Benjoo *, [NewAge] Maksimo007, raygun gothic, GENTRY, (PPI/ParaUnit)169PD, (PPI/ParaUnit)169PD