It's been several years since the defeat of Majin Buu. Making his return from Dragon Ball, Emperor Pilaf gathers the 7 dragonballs but is interupted by Goku and out of frustration, wishes him to be a child so that he could have a chance to beat him up for all the times he has foiled his evil plans. Eventually Goku, Pan, and Trunks travel the galaxy to collect the 7 Black Star Dragonballs for the purpose to saving the Earth which is set to explode in 1 year. In this part of the series, Goku is seen as a child for the majority of the show except for when goes Super Saiyan 4 which reverts him back to his adult form. He is seen as an adult only in the last episode where his granddaughter Pan Sees him during a martial arts tournament.
The Three Major Villains in DragonBall GT are: Baby (Bebi), Super 17, and Omega Shenron