I think Gallagher is a very good player who has just come off the back of an excellent season.
I remember commenting on here about him after we had played West Brom away when Gallagher was on loan to them from Chelsea. I was really impressed at the way he was able to more than hold his own against us, despite being so young and in such a poor team.
I'm not convinced that Bentancur is a great number 8 option for us really in the way that Ange likes his to play (especially how he wants us when we're off the ball). Bergvall is a kid who will likely need another couple of years (at least) before he would be considered a starting first team player).
Signing Gallagher as a good, young, proven in the PL, English player makes a hell of a lot of sense.