The official Japanese Pokémon website has revealed the Pokémon White Mega Gengar Campaign (ポケモン　白いメガゲンガーキャンペーン) which will take place from September 13th, 2013 until October 14th, 2014. A Shiny Level 25 Gengar distribution is the highlight of the campaign. The Gengar knows the following moves: Shadow Ball, Sludge Wave, Confuse Ray and Astonish. It is holding a Gengarite allowing it to Mega Evolve and is stored in a Dusk Ball. Sludge Wave is a move that Gengar can't normally learn.

M Gengar EX (メガゲンガーEX) TCG Promo Card

Patrons of the Pokémon Centers who spend 1500yen or more before taxes on in-store merchandise can receive the Mega Gengar EX (メガゲンガーEX) 079 promo card.

Card Name #79: Mega Gengar EX (MゲンガーEX) Card Type Mega (Pokémon) Energy Type Species: Gengar [Cards] Evolves From: Gengar HP: 210 Weakness: x2 Resistance: -20 Retreat Cost: Rarity: None Attack: Hollow Geist (ホロウガイスト) - 120 Your opponent's Active Pokemon is now Poisoned and Confused. (相手のバトルポケモンをどくとこんらんにする。) Artist(s): 5ban Graphics

Pokémon White Mega Gengar Tretta Lottery

Pokémon Tretta players that purchase specially marked merchandise totaling more than 1500 yen at a participating store will receive a lottery ticket to get a special White Mega Gengar Tretta pack. 1000 lucky lottery participants will receive the pack which includes: a Monster Collection Mega Gengar (Alternate colored) figure [モンスターコレクション　メガゲンガー（色違い）], Alternate Colored Mega Gengar Pokémon Mega Ring (ポケモンメガリング　色違いメガゲンガーカラー) and a Special Mega Evolution Toretta "Gengar" (White Metallic ver.) [特別なメガシンカトレッタ「ゲンガー」 （ホワイトメタリックver]

Sources: http://www.pokemon.co.jp/ex/white-gengar/campaign.html and http://www.pokemon.jp/#/contents/game/pickup_07/