In Dragon Ball GT, Goku as a kid still had the ability to not only transform into a Super Saiyan, but the Z-Fighter was also able to become a Super Saiyan 2 and Super Saiyan 3. Of course ...

This also means Goku is now working his way towards bringing back his rarest and coolest transformation, Super Saiyan 3, which at this point in the timeline is the strongest he’ll be able to go.