Its aizen's confidence, because no doubt he is immortal by the time he is done so for him the fact that his plan included that means why should he fear anything after all he will be immortal most he will need to do is out live everyone ever and start again.
Gin could go to the Jujutsu Kaisen world before Gojo and Fushiguro Toji's fight and make a bet with Gojo that if he loses a battle against someone in a week he will copy their eyes, unlimited and family lineage.
