No. Life Drain is an attack that places damage counters. Pluspower only adds to base damage. And since effects of attacks technically have no base damage, so it won't work.
No, regardless of what you think PlusPower means, you cannot deal 10 damage more than no damage at all. You have to be dealing damage already for PlusPower to add anything to it. If there is no Base Damage, or nothing that says "Deal X damage.", like the Splatter Piplup from MD. It deals damage, but PlusPower only applies to it if you deal the damage to the active. PlusPower doesn't add damage if you deal the damage to bench Pokemon. It works the same way if you use an attack like Lucario from DP, damage to the active AND the bench, PlusPower only adds 10 to the damage dealt to the active.
Ok so that kind of got off topic, but I seem to do that alot. So no, PlusPower would not add to Gengars attack. Sorry for rambling.