Fight is so difficult to handicap because we have no idea what the combo of mass bulking/more than 3 years off will do to Jon, for better or worse. He could be a wrestling/GNP beast that takes Gane down and pounds him through the canvas. Or he could be too slow to close distance, slower and less powerful than Gane in his strikes, and get picked apart by the guy used to carrying his muscle mass.
Having never seen Jon spar, let alone fight at the size he is (looks huge in pics I've seen) I can't feel comfortable picking a winner.