What happened to Goku after he left the Earth at the end of Dragon Ball GT? 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.