See I'm of the opinion that Gallagher holds none of the cards. The scenarios for me are:
He refuses to play, therefore threatening his chances of making the Euros squad. There's options at centre back and he's not established enough to be a nailed on starter.
Factoring in that he's, effectively, got 1.5 years left on a deal we can command a pretty sizeable fee for our club captain and full international player. As much as I don't think we'd be able to get someone of the same quality in at such short notice, we probably would accept a fee.
He realises that he does not, in fact, have a gun to his head. Plays a few more games, extends his contract, goes to the Euros, pockets big Kane in June and we all live happily ever after.