Technically it both is and isn't, it's alternate time line canon. I consider a lot of things cannon that a lot of people wouldn't, and in GT buu was actually pretty nice of a guy. Supers on the other hand.... his bit of a basterd... which I just hate really. and technically GT was cannon entirely until they decided to do supers, so it's now pretty much considered alternate time line cannon (which is possible because of the fact of future trunks back in dbz)