It's 20 floors and Gengar will stay as a Pokemon, but Gardevoir will be freed.
I don't think Ninetales would allow Gengar to go back to being a human, even if she had the power to do so. He screwed up so much as a human, there's no way a vengeful kitsune like her would undo his transformation.
Even though I love Gengar to itty bitty pieces, he doesn't deserve to be human again. Besides. Who would want to go back to being a human anyway? Being a Pokemon rocks! =D