Gohan hasn't been proven to be the strongest without any doubt, he fought relatively evenly with Goku at his best with Ultra Instinct, but Broly could dwarf all other fighters if he controls himself and makes the most out of his own potential. Even as a base Super Saiyan, Goku thought that Broly could be stronger than Beerus or at least put up a really good fight against him compared to Goku.