Priests would be equally overpowered with Monotheism, perhaps even moreso. 100% health potions with GG's massive HP boosts? Madness. I could maybe see Priests getting a small (+2 or +3) piety boost whenever they quaff a potion; not enough to eliminate the penalty from Dracul or GG, but enough so that the penalty isn't so devastating. Moreso for Dracul, whose penalty is -5 piety, than GG, who takes off 10 piety for each potion, but like I said 100% health potions with GG could quite easily be broken. (Personally, I think that if Monotheism is going to stay, with any class, it'll need to be toned down - probably not zero piety loss, but perhaps limiting all piety loss to -1.)
In terms of their anti-undead ability, it might be worth considering to give Priests a toned down but broader 'slay evil' ability, something like +40% to undead and any monster that poisons or mana burns.