Two limited-time Pokémon distributions will be held at GameStop in the U.S. for Pokémon X and Pokémon Y starting next month. These distributions will allow you to avail a Shiny Gengar and Diancie for the two games.
The Shiny Gengar will be available first, from October 13th – 26th, and will come holding a Gengarite Mega Stone, which will allow you to Mega Evolve it. Meanwhile, Diancie will be available from October 27th – November 16th. A Mega Stone for Diancie is included in Pokémon Omega Ruby and Alpha Sapphire, so transferring the Pokémon to those games will allow you to Mega Evolve it as well.
The two distribution events are being held in celebration of the launch of the Pokémon Trading Card Game XY: Phantom Forces expansion, which will introduce Gengar-EX, Mega Gengar-EX and Diancie to the TCG. It will be available starting November 5th.