As a massive Dragon Ball fan, I am getting tired of the whole "GT isn't canon!" people who feel that way because they thought the show sucked. The way I see it:
-"Toriyama didn't have anything to do with the series, not canon!"
He designed the Super Saiyan 4 form, penned the entire beginning arc with Dr. Myuu, had the idea of Pan becoming Goku's sidekick, and even supervised the storyboard artists for the first 40 episodes. He might have not been as immersed in the project like in the first two series, but he still was a driving force to get GT moving.
-"There was no manga counterpart, not canon!"
There have been many anime that strayed far away from their manga counterparts and still attained huge popularity, including the first Fullmetal Alchemist series, Elfen Lied, and Evangelion. Staying in tune with the manga was what caused Dragon Ball and Dragon Ball Z (especially the latter) to have filler episodes and pacing problems, not to mention many inconsistency. It was so bad that Toei made Dragon Ball Kai to get rid of the these issues, basically saying "fuck you filler". Because GT had no manga to be restricted with in terms of deadlines in production, it had more free range in creativity and flexibility with its universe.
-"Toriyama has said GT isn't canon, not canon!"
He has not said this at all and I have google'd thousands of times to see if he actually said it. People claim that the translation in his introduction of the Dragon Box DVD set (
http://www.kanzenshuu.com/forum/viewtopic.php?t=5411) means that "side-story" indicates "not canon". A side story simply means that it deviates from the main timeline in a particular way, not that the events take place in another universe (like how Rogue One is a Star Wars story that takes place during the rebellion) Until I hear the words "yes these guys are correct" or "yeah GT has nothing to do with the overall story" out of Toriyama's mouth, there's no proof at all.
-"The writing sucks, not canon!"
Obviously a fanboys response to "Wow this series is nothing like Z! It has great character development, episodic monster-of-the-day stories, epic battles, and lame dubbing! Stuff that have all been in the previous 2 series! It sucks!"
Sorry for the autism, just needed to point out how stupid the DB community can be when it comes to this debate.