Are you dense? Goku even said it himself, when he first saw majin buu he could've destroyed him. He just wanted his sons to step up as the defenders of earth since he won't always be around. Goku went toe to toe with Kid Buu, which was arguably much more dangerous then the buu Gohan went up against, as a ssj 2. And don't forget, Gohan was nearly killed by the fat buu while Goku was slapping him around as a ssj 3.