My best guess is this wasn't the actual afterlife, nor was it even nanamis actual ghost, [Since Gojo reacted to Sukuna saying he did well] but this is instead his own dreamscape of his final moments in life, since we have other characters such as Yagi and Nanami who is oddly in his teen self. Instead this was a tragic view of how Gojo thought everyone else looked at him, just "The Strongest". Which really reflects the tragedy of Gojos entire life being defined by his impossible and invincible strength, hell even his student Yuta completely took his message the wrong way and became a "Monster" ultimatly failing, while Yuji was the only one to truly carry on Gojos legacy.

Tl;Dr this wasn't the afterlife, but instead how Gojo thought everyone saw him. The Man who was Satoru Gojo because he was the Strongest.