DBGT is completely non-canon.
Which doesn't necessarily make it a bad thing. Some of those awesome DBZ movies were non-canon.
DBGT was crap though. Don't get me wrong. There were some great ideas: Traveling space to look for the dragon balls, tuffles getting revenge on the saiyans, the dragon balls being overused and turning against the good guys, etc. However, none of them were executed properly. The action and animations were piss-poor compared to DBZ, or even the original Dragon Ball, and generally there were too many plot holes.