Dragon Ball GT takes place several years after Dragon Ball Z. The series mainly focused on fanfare, and going back to its Dragon Ball roots merging with Dragon Ball Z.
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