Power Claw states "This attack's damage isn't affected" by Poke-Powers, meaning Powers that would try to change Power Claw's damage will have no effect.
Fainting Spell does not have anything to do with Power Claw's damage. Fainting Spell triggers when Gengar gets KOed, which happens after the damage from Power Claw has already been done. The attack is over by then, so Fainting Spell would trigger in this instance.