lets face it, GT is stupid, shouldnt ever be taken seriously.
on top of which, i never heard that gogetta was SSJ2 in the movie, i always thought both he, and vegetto only went ssj. the lightning isnt just an indicator of ssj2, if you notice through dbz, whenever anyone generates enough energy, they can cause lightning, just not in such abundance.
i think both vegetto and gogetta's power is so high that it would cause lightning no matter which "stage" they were in.
that being said, it was stated many many times through dbz, that goku and vegeta were the most powerful of fighters, and if they fused the results would be insane. when vegetto was born, he was the most powerful fighter in the universe beyond ALL reckoning.
i feel its a fairly safe bet, that if gogetta learned to go ssj3, his power would be off the chart, much like ssj4 (which is the worst transformation ever).