Giorno's Gold Experience Requiem's ability seems pretty powerful but it cannot defeat Dio's Over Heaven stand. Dio is a god and his stand has the ability to overwrite reality. This means that Dio can simply make Giorno's actions and willpower non-existent and defeat him easily. Additionally, Dio's stand has great power and speed, giving him an advantage in combat. Giorno's speed may be a match for Dio's but it won't be enough to defeat him in the end.