Gio is getting worse while Leake is in his prime and Chen is maintaining. Move Gio to the Yankees and he's a mid 4 to low 5 ERA pitcher. That is simply not worth 12 million, nor worth giving up Miller for. They Yankees don't want Betances closing. Right now they can bring him in to the game whenever they want and use him to get to late inning match ups they want.
Why is Gio magically a 4 or 5 ERA pitcher in NY? He has upped his groundball rate as his fastball declined and his FIP is still much better than his ERA. He suppresses homers and still strikes out more than 8 per 9 innings. He also would have led the Yankees staff in innings last season. Chen has a much worse FIP, much higher HR/9, and lower K/9. He is only better in innings than Gio. Gio is actually younger than Chen as well. The only thing that Leake has is two years of age, but hewas worse by every metric. He even was worse at holding back homers in SF than in Cincy.
Again, unless the Yankees pursue Cueto, Gio is a better option for 3 years at 12 Million a season, and if money is a factor, than Gio seems quite appealing.