Okay, so the two sides are:
"Grand Priest should be Low 2-C scaling from Infinite Zamasu"
And:
"We have no proof that none of the High 3-A characters can affect space-time and are stronger than Infinite Zamasu"
I am of the opinion of the former.
Let's analyze evidence:
- 1) So, as we all know, Infinite Zamasu has a blatant Low 2-C feat. Encompassing the universe (And later multiverse if not stopped) across space-time.
Let's see how Beerus and Whis react to this:
https://animefrost.us/anime/dragon-ball-super/0/67/
(Apologies for linking to Fansub Site. Couldn't find the scene on youtube)
Scene happens at 7:22 minutes.
Whis: Hmm... I sense... A flow of unpleasant energy that makes my skin tingle.
Beerus: It's Zamasu. Something happened in the future. It's having an effect on the present.
Notably, both Beerus and Whis are concerned and viewing Infinite Zamasu as a definite problem / threat. This is notable because up until this point, Beerus has been portrayed as completely non-nonchalant and casual. He didn't fear any of the Super Villains up until that point.
So no doubt Infinite Zamasu is at the very least a significant threat to Beerus, if not stronger (Which we assumed to be the case given him having the best feat in Super besides Zeno).
Now, Jiren arrives and changes everything.
- 2) Statements about Jiren's power:
https://www.youtube.com/watch?v=h7ZxDqukDxU
Supreme Kai: This power feels different from anyone we've ever faced before. He's strong. Plain and simple.
So Supreme Kai says that Jiren's power is different than anything he and the U7 have faced
because he is stronger.
Whis: I daresay... Like a God of Destruction. He is the one who has reached that state. Perhaps even surpassed it. "There is a universe where lives a mortal even a God of Destruction can't defeat." It appears the rumor is true.
And Whis says that Jiren's power is like that of a God of Destruction,
if not stronger.
By the way, this happens when Jiren is still really suppressed tanking the Genki Dama.
Then Goku reaches his new, Ultra Instinct Form and fights a serious Jiren.
- 3) The Gods' reaction to Goku and Jiren:
https://youtu.be/yasZEXWUrw8?t=185
Khai: I can't believe someone who could gain on Jiren this much even exists!
Belmod: It's not possible! It can't be possible!
So Khai is absolutely shocked someone can match Jiren and Belmod says it's not possible.
By the way,
Jiren is stronger than Belmod, by Belmod's own word.
https://www.youtube.com/watch?v=SvxLbcJuYB0
Look at all the GoDs' and Kais' reaction to Ultra Instinct Goku. They are shocked that he is so strong. Beerus is left with his mouth agape.
Beerus: Ultra Instinct...
[Champa is utterly shocked, other GoDs get surprised]
Anat: I see. Those movements... But, for a mortal like Son Goku...? That is a state not even a god can easily attain.
Not only that, but the Ultra Instinct level is something that the GoDs have a difficulty reaching. The Angels have no problem with it, but it is really impressive to the GoDs.
With all of this in mind, here is my conclusion:
Jiren and Ultra Instinct Goku are undoubtedly stronger than the Gods of Destruction, even if not by leaps and bounds. But they are still weaker than the Angels and Gods of Destruction.
Shin says that Jiren's power is completely different than anything that the Universe 7 has ever faced, because he is stronger than them. To that, Whis agrees, and states that he is on the level of a God of Destruction, if not stronger.
Beerus is utterly shocked at Ultra Instinct Goku and Jiren's power, when he was previously only concerned by Infinite Zamasu.
The conclusion is simple:
Jiren and Goku, and by extension the Angels and Grand Priest, are stronger than Infinite Zamasu.
It is likely that Infinite Zamasu is on the level of a God, if not a bit weaker. He is at the most on the same level.
Meanwhile, Jiren and Ultra Instinct Goku are stronger, even if not immensely strong.
The Angels are even stronger, and Grand Priest is above even that.
All the Gods of Destruction should be changed to Low 2-C. The justification should be "Any God of Destruction can destroy the universe even when holding back immensely, and should any two GoDs fight, the collateral damage will result in the destruction of their respective universes. Should at least be comparable to Infinite Zamasu"
Jiren and Ultra Instinct Goku should be Low 2-C too. The justification should be: "Even when holding back, Jiren was stated by Supreme Kai to be more powerful than anyone he and the Universe 7 warriors have ever faced, which should make him more powerful than Infinite Zamasu. Stated to have reached the level of a God of Destruction, if not surpassed it completely. His power let Beerus completely shocked, when he was previously only concerned by Infinite Zamasu's power. Even when suppressed, his Ki alone shook the entirety of the World of Void, a dimension of infinite size"
All the Angels should be Low 2-C, with the reasoning being (Much stronger than any God of Destruction, and stronger than Jiren and Ultra Instinct Goku as well)
And Grand Priest should be At least Low 2-C for being far above the Angels and inferior to only Zeno and three other unknowns.