If they were the same size: Gamboa 120-108.
That's the only answer I can give you because 1)Rios can't make 140 anymore 2)Gamboa is not a light welterweight.
Rios fights -and has always fought- 2-3 weight classes above Gamboa. You might as well ask: Rios vs. Golovkin who wins?
Probably rios, I think his size would wear down yuri over the course of the fight..
Would have loved to seen this a few years ago when it was suppose to happen at 135