I doubt that time is going to skip ahead, that doesn't really seem like the author's style to me. And from what I'd assumed after this book, the rebellion seems to be moving along quickly enough that everything can happen just afterwards. District 13, if it does still have nuclear technology, should spur action ahead quickly enough. While I don't think that anyone will actually USE nuclear weapons, (because that wouldn't be a good moral, and would be pretty boring as well,) the nuclear threat will create some sort of action.
As for who Katniss ends up with, I don't really see it being Gale or Peeta. The bond between her and Peeta is more symbolic I think, created in order to survive the Games and appease the Governor or whatever his title is. Likewise, Gale seems to fit almost perfectly the sacrificial role- he'll come to repent his cold and angry role by saving Katniss & Friends, probably costing him his life.
Those are just my predictions though, I'm probably way off.