there are 3 possibilities:
1- Gojo is back; I remember that Ui Ui took Gojo's body to Ieiri Shoko, she's the best when it comes to RCT, so I think this is most possible option.
2- Kenjaku took over Gojo's body; even though I didn't think Kenjaku would die that easily, but I don't think he can take over Gojo's body, because Shoko was taking care of Gojo's corpse, but if Sukuna dies then who will activate the merger, so I think there's slim chance that Kenjaku has taken over Gojo's body.
3- Sukuna's hallucinating; I don't think this is possible, because Sukuna is still very much alive, he was about to expand his domain.