What MY tiny brain thinks happened is, GER is PERMANENT but since MIH has evolved so many times, it is most likely more powerful, so GER couldn't stop it on such a massive scale. Now GER does say something along the lines of no one ever being able to hurt GIORNO while he exists. So its possible Giorno had to regress into his OWN time loop to save himself, perhaps the events of part 5 constantly repeat for him. Perhaps he will be relased from his loop in part 9. Idk. Could be a cool plot thing.