I have heard about the theory of that GDL is actually Belarusian also called "Litvinism" lots of times. I think it is weird,makes as much sense as Finns claiming the Swedish Empire was actually founded by Finns (ofc theres no such theory), nationalistic and i know its not true,but where is the supposed evidence for this theory which says that GDL is actually a Belarusian state....
Depending what you mean. If you're asking about population and territories than yes it was mostly what is now Belarus and Lithuanian people were definitely not majority. But if ruling class was Lithuanian