Well, you pretty much just proved my point. It's widely believed to be non-canon, because canon, for the most part, is based on belief.
"Canon" is a tricky thing in any franchise, and pretty much just comes down to each fan's choice of when to say "when." And if you ask most hardcore DB fans, they'll tell you GT doesn't count. It's on the same level as the DBZ movies. Basically, it has no filler because it IS filler.
DBZ is closer to something like Harry Potter than Spider-Man, because DBZ comes from the mind of a singular author, and is not a never ending story with hundreds of writers. So by that logic, the only undisputably canon part of the story is officially over when Toriyama stops writing. From that point on, it's up to each fan to pick and choose what they like.