Toriyama is rather insistent that everything in Dragonball happened, but it didn't necessarily happen in his personal continuity(He specifically referred to GT as a "Fun Side Story" which instantly made it more enjoyable).

I've been going with the Xenoverse Explanation-the Main Continuity is the Manga and the two recent movies/Super(Possibly both, depending on how the retellings and expansions go), and the earlier movies and GT take place in alternate timelines that were created by Trunks and Cell fracturing space time with their repeated time travel.