Gamboa beats Broner. It's not easy work though, no matter who you see winning.
Size is a good thing to have as an advantage and Broner does. However, being more skilled, faster, far more experienced and simply a better fighter is what Gamboa has in his favour. Even moving up, I'd also be confident in saying Gamboa hits harder.
Gamboa being 'chinny' isn't really anything, he was often knocked off balance because of his reckless nature, not hurt. That reckless style has been balanced out with patience and smarts over the last few years.