Gengar is receiving errata for its Poke-POWER, "Fainting Spell". Fainting Spell affects your opponent's Attacking Pokemon, not your Defending Pokemon. The correct card text should read, "Once during your opponent's turn, if Gengar would be Knocked Out by damage from an attack, you may flip a coin. If heads, the Attacking Pokémon is Knocked Out."
Source: Organized Play News (2008-11-07)