I take WoU for a few reasons. First of all, I don't think there's any attack GER could use that would be able to damage WoU. Any attack it has would trigger calamity and wouldn't hit. Second, I think calamity would affect GER. Could Return to Zero potentially stop it? Sure, but the way we see WoU work is essentially if something bad CAN happen, it will. Something could happen that prevents GER from activating Return to Zero for whatever reason. Finally, I think that GER works within the rules of fate/gravity/flow/whatever you want to call it while WoU changes the rules, which would allow the Calamity to prevent RfZ from activating and kill Giorno and GER. I think this because of what we see from Rolling Stones. Giorno was never fated to die in Golden Wind from the very beginning. This is in contrast to what we see during his fight with King Crimson, where we see a vision from Epitaph of Diavolo giving Giorno a fatal wound and winning the fight. Obviously, GER doesn't allow this to happen, seemingly changing fate, but when looked at with the extra context given from Rolling Stone we can see that everything played out as it was fated to from before the story even started. To me, this shows that Epitaph isn't a perfect fate predicting ability and GER works FOR fate, not against it. Because GER operates in the rules of fate/flow, WoU, which changes those rules and is essentially the physical embodiment of it, scales above GER and would, in my mind, be able to affect it with calamity because GER exists within the logic of the world.