The problem here isn't even Sukuna's cut, which even, seeing where the blood is coming from, seems to have been cut all over the place, that is, the head will fly off. But that's the least of it, since with the brain intact it will regenerate almost instantaneously.
The biggest problem here is the fact that Gojo is now inside the Malevolent Sanctuary which was not annulled, as said in the cap, this would decide the winner, basically, now Gojo is in Mizushi's guaranteed hit area, at any moment Sukuna can literally turn Gojo into powder (remember shibuya, right, it wasn't even a brick, Makora turned into a sugar cube), if Sukuna uses it, there will be no reversal or Mugen to save.
At this point, not even a black void or anything like that that was being theorized will be very useful here for Gojo, there's no point in a powerful attack being that he's inside an expansion, which is the apex of a Jujutsu technique, the level expansion they have is almost absolute in a battle, it's entered = died. Unless he pulls off an extremely convenient technique to cancel Mizushi, the only way for Gojo to make it out alive is to flee the expansion area.
Problem is that Sukuna's insane amount of energy was said, even if Gojo runs away, he probably manages to expand the expansion again, that is, it goes back to 0, the fight is basically already decided as said. The two options now are for Sukuna himself to close the expansion in order to “play” more or for Gojo to use a very convenient technique that nullifies Mizushi. That's because he didn't even start using shadows, or even his other techniques, like fire, the situation for Gojo right now is pretty bad.