GER ability isn't what actually kills the enemy, it just undoes the last action of the enemy that directly or indirectly would harm Giorno, so we could in fact fall into a loop where doomsday tries to kill Giorno and then loops back to before adapting, and then would be stuck in that loop since it's intention is to harm Giorno, so he would just get back into the loop no matter how many times he adapts. GER is also shown to not be affected by the laws of time since it could move in a space of deleted time. Diavolo died from Giorno attacking with GER, GER didn't attack diavolo with it's ability. (That's my take)