"It has been revealed that Japan is to receive a special Gengar event. This event provides a shiny Gengar as part of their "White Gengar" promotion for Halloween. This Gengar is Level 25, comes in a Dusk Ball, and has the moves Shadow Ball, Sludge Wave (which it can't normally have), Confuse Ray and Astonish. It comes holding the Gengarite. This Gengar is being given at various, currently unconfirmed, stores across Japan from September 13th to October 14th."
Hopefully we get this outside of Japan.
Japan isn't the only country that celebrates Halloween so maybe that somehow raises the chances.