GT was not canon nor was it made by Akira Toriyama. He's been trying to erase it's existence as of late with all the movies that have been coming out recently by giving Goku a stronger form than the atrocious ape looking Super Saiyan 4 with the whole Super Saiyan God.
It'll be interesting to see Toriyama make a new SS4 for this new series.
The story is taking place after the Majin Buu saga so we can still have Goku as the main character.