his ability in the lore isn't just "resurrection" per say, it's more of "if something is deemed impossible for him to do, a miracle will happen and make it possible". he shud be able to resurrect infinitely. so long as it's "impossible" for him to win, he'll come back stronger in an attempt to make that outcome possible
according to a QnA with the author, gerard has a Quincy Core. if it get's destroyed his powers won't work again and he'll be defeated for good, but his body was torn to pieces and it still wasn't destroyed so nobody knows for sure where it is or how small it is. his core was never destroyed in the show. as far as we r concerned only confirmed canon way to kill him for good is yhwach taking his powers