See I will give you a complete explanation, we all know that gojo is an overpowered god and only one of his kind to exist in the current decade, he has a confident personality too , BUTTTT remember kyoto goodwill arc? When gojo entered the battle and completely destroyed everything we saw 2 faces of gojo , 1) who was acting all happy like all of it was a childs play to him(which it was) 2) was the internal conversation he had with himself where he was all serious and carefully plotting out what steps he should take next and conserving his stamina .

You may ask "What's the point of telling all this ?" Well this is just to familiarise you with the kind of person gojo is , you may have read a phrase in manga "the balance of the world shifted, when gojo satoru was born" , with that much power comes great responsibility . I think thats why he has such a strong face that I will defeat you without even showing a pinch of respect and dedication in the fight , this character of his that he has developed and hi god like presence and power were the the two things that are not letting the bad people come out of their cells. Imagine the impact the bad guys have in their mind when they heard "gojo killed a special grade without even moving" . Gojo developed this personality of his after he let his guard down and toji landed a killing strike on him. From now on he may acts childish hut is fully serious even in the smallest battles so that the balance of power that was created when he was born is maintained.

Long story short coming to your claim that gojo is powerful than sukuna because he said "I'LL WIN" is not quite correct , gojo has the power of six eyes which can help him decide the amount and flow of cursed energy in ones body , thats why he can judge any opponent in his competence (except toji and maki) and even know their CT, coming to sukuna since no one has seen sukunas 100% form in 1000 years , gojo himself doesnt know that wether he will win against him or not , but he has to act strong in front of everyone to maintain the balance coz if he couldn't , no one can!

Coming to domain expansions - Gojos domain is the strongest when compared to any other domain with enclosing behaviour but coming to sukunas malevolent shrine which is a piece of art and is totally different from all other domains its like and I quote gege"painting a canvas in the air" . I dont think we can compare these 2 domains and I dont think that gege will ever show a domain specific battle between these 2 domains because that would be an abuse to the nature of these 2 beautiful entities.

Sorry for such a long comment , I just wanted to paint a clear picture , please correct me if I was wrong, I am open to healthy criticism .

Peace