Fans have argued about whether GT is canon for decades, so the discussion is unlikely to be settled anytime soon. Part of the issue is that the word “canon” can sometimes get muddled, because its use by fandoms is often not very precise. In a case such as that of GT, it’s worth defining what canon actually is. The word “canon” is derived from the Greek word kanōn, meaning “rule.” In the most basic sense of the word, canon is the “rule” rather than the exception.
In academia, the word canon is often used to refer to holy scriptures that are seen as the “true” works of scripture, as in “the biblical canon.” This term has come to be extended into fiction fandoms to mean something quite similar: the “sacred” and genuine pieces of literature that make up the true work of said fictional universe. This is especially important in series with many spin-offs and fan fictions, because fans often want to differentiate between what is genuinely the work of the creator in the universe they intended, and what is an off-shoot of that universe created by someone else. In other words, the rule instead of the exception.
In this sense, Dragon Ball GT can never be considered genuinely canon, because it’s not a work that the “God” of the Dragon Ball Universe, Akira Toriyama, personally created.
If canon is considered a work that is a “true” part of the series, it should at the very least be crafted by the actual creator of the series. Unfortunately, as Toriyama said himself, he didn’t put very much into the production of GT, instead choosing to leave “everything, including the story, up to the people on the anime staff.” This is in contrast to his work on other, more recent series, like Super and DAIMA. Toriyama was heavily involved in the writing, character designs and concepts in both Super and DAIMA, making them the most definitely canon pieces of Dragon Ball media to date other than the series’ original run.