What happened to Goku in those 100 years after he left the Earth (by which Pan was already an old woman)? Did Goku die or not?
A lot can be said about the last episode.
You could argue that Goku went to train with Shenlon, you could argue that he offered his own life for that last wish, you could argue that it was all an illusion, seeing Pan holding his shirt after he had supposedly left, you could argue that he became one with the Dragon Balls/Shenlon, you could argue that he'd become immortal.
This list goes on and it will remain to your own imagination what you believe has happened to Goku, as it was never thoroughly explained in the anime. I guess if anything, we could agree that Goku had surpassed human existence and entered a state different from being alive or dead as we know them.
Goku was dead to begin with at the end of DB GT, and when asked if they should revive him back, he didn't let them do that. Instead, he feared that there are more dangerous and strong fighters in the world, and he needed to practice and therefore went with Shenron to practice and live a peaceful life for sometimes.
He did mention he will come back when the time is right, and since then there were rumours that a new season will come. But at the moment, there isn't any and GT weren't part of the original Dragon Ball series.
Dragon Ball GT is an alternate timeline.
After Super or maybe later in the series, they might reintroduce Super Saiyan 4 as the "Lost Form" or the "Forgotten Form" or something like that.
My other theory is that maybe the people who created Dragon Ball will create a story where GT Goku and DBS Goku will eventually meet up... when the time is right.
I think that when GOKU went with shenron and meet his old friends for farewell he know that his time has come to say goodbye .And when goku said to shenron that he was feeling very tired than ever before it's because he was dying and disappears .Also the reason pan saw goku's dress in the end because he was dead😪 And Vegeta therefore said pan to take care of them
If Dragon Ball Super ends after the Tournament of Power, and if Universe 7 wins, Goku would have asked Super Shenron to reconstruct the destroyed universe and erase the memory of all the people of Universe 7 from meeting Beerus for the first time at the end of the tournament. Then they would have lived a normal life. And after this, Dragon Ball GT is started.
At the end of Dragon Ball GT, Shenron glows his eyes and due to this, Goku is revived and Shenron asks Goku to come with him. And Goku says yes to Shenron, which was an unexpected behaviour shown by him. After Shenron revived Goku, he must have given his memory back and I think Shenron made him immortal and given him a god soul because of his good deeds. And due to this, in the farewell tour, Goku was able to go to hell to meet Piccolo.
And in the ending of Dragon Ball GT, Goku meets Goku junior and disappears. And at last, Goku was seen by Pan in World Martial Arts tournament after 100 years. He was checking if the earth's protection was in good hands. And at last, Goku fly with his power pole "until we meet again".
I think Goku was not dead, he was trained by other gods and at last made him an immortal saiyan god.