The official Pokémon website has announced that players of Pokémon X and Pokémon Y can receive a Shiny Gengar holding a Gengarite Mega Stone when they pick up a special code card from participating GameStop stores between October 13th, 2014 and October 26th, 2014. This Shiny Gengar can Mega Evolve into Shiny Mega Gengar during battles.

Also available at participating GameStop locations starting October 13th, 2014 is a Pokémon TCG: Gengar Spirit Link 2-pack. It includes two booster packs from the Pokémon TCG: XY Furious Fists expansion, as well as the Gengar Spirit Link Pokémon Trainer card that debuts in the next Pokémon TCG expansion, XY Phantom Forces.

Mythical Pokémon Diancie

Players of Pokémon X and Pokémon Y can then return to participating GameStop locations between October 27th, 2014 and November 16th, 2014 for a serial code card to receive the Mythical Pokémon Diancie. After receiving Diancie, it can be transferred into Pokémon Omega Ruby or Pokémon Alpha Sapphire when those games launch on November 21st, 2014 for the Nintendo 3DS. Once the Diancite Mega Stone has been obtained in the new games, players can Mega Evolve Diancie during battles!

Source: http://www.pokemon.com/us/pokemon-news/a-pokemon-autumn-to-remember/