>to get a Gengar, go the the Global trade Centee and ask for someone to trade you a Haunter, this will evolve into a Gengar. You could also trade for a Haunter via Friend Code Wi-Fi trading or you could find a Psychic Trainer named Bryce who has a Gengar who is within the first floor of the Victory Road.
>Route 205 North, Route 210 North, Route 225, Route 227, Route 228, Ravaged Path, Oreburgh Gate, Eterna City, Great Marsh, Mt. Coronet (Fish With Good Rod).
These are all the places it is located. When it says Mt.Coronet it probably means fish in that town where Cythina Granmother lives. Thats where I found my Barboach.