The only thing that was offscreened was Gojo's body being pierced, a single instant; we saw everything else. Sukuna didn't heal anything. He's still missing a hand and has the same wounds he had right after Hollow Purple. Just like Gojo's injuries are still the same mid-healed ones they were on that panel, and his legs are standing upright.
It was only moments before the Hollow Purple thing that Sukuna got the adaptation he wanted from Maho, but still tried to play it safe and got ousmarted by Gojo who managed to detonate Purple. However, Gojo failed to put the pieces of Sukuna's strategy together and was probably off guard coming out of Hollow Purple after seeing Sukuna's state. Sukuna went for the kill with his first "adapted" slash that he barely had gotten a hold of moments earlier when his gambit paid off.
There is no offscreen. Gojo died the same instant last chapter ended: a slash simply went through his infinity. That's why the first thing we saw was him in "heaven", because it was as abrupt and instantenous for him as it was for us.
Both characters came out looking great from the fight.
- Gojo got more than ten chapters of showing off his full potential and everything he can offer in every aspect, and got Sukuna to praise his strength and claim he will remember his name forever. And let's remember he did make Sukuna feel nervous after the black flashes. Even if he ultimately knows he had no way of winning, he is still the only character so far who's not low diff material for Sukuna, actually got him to strategize and even injured him quite heavily. Not to mention he kept doing "impossible" things and Sukuna actually learnt a trick from him.
- Sukuna outplayed him and killed him, and got confirmed as being the stronger (after all, he was always portrayed as the strongest in history. A history that included Gojo's ancestors with his same skill sets). All this, "ultimately he still needed Maho to win" is what we get from people skimming over the fight, specially over the starting parts, during which Gojo was surprised Sukuna was banking so much on Maho instead of going with much more secure/efficient strategies. He could bypass infinity just through domain expansion and domain amplification, and we know he's still hiding stuff we don't know about. Had he not had Ten Shadows, he would have simply gone with a different strategy.
Good chapter, good death, very much in tune with the tone of the series.
