That's a bit of a stretch. Civil War couldn't reach AoU or even IM 3, so I doubt GOTG 2 will reach JW totals. I'm certain it pass 1 billion, but I don't see it bringing in 2x GOTG 1. Anything is possible with Disney though. The new trailer has certainly got every viewer hooked, line & sinker. I just really hope Gunn will come back for GOTG 3. I can't imagine it being as good without him.
I think that GotG2 will do better than Civil War, and has a decent shot at topping AoU and IM3, due to the franchise's popularity now. The first GotG was held back by the fact that it was an unknown entity at the time. I had plenty of friends that did not see it as a result of that. Add in a more popular Chris Pratt for the ladies and Baby Groot for the kids, and this film has more widespread appeal potential than the first one did.
The only major competitor for GotG2 in May will potentially be the latest Pirate's of the Carribean flick two and a half weeks later (why Disney would do two major releases in the same month is a bit puzzling), but that franchise is not what it used to be.
I agree with you re: Gunn. If he leaves it will be tough to replace what he has pulled together with the franchise.