In general, I'm very happy to have this code ported to use gesture detector so we don't need the shim anymore.

I'd prefer to do it with a single GestureDetector object if possible. Or maybe one for the fullscreen mode and one for edit mode?  Its been a while since I wrote the gesture detector code, though, so I'm not positive what the performance implications are.  But my first impulse would be to create the detector higher up in the document tree.

Also, do you have any corresponding calls to stopDetecting()?  Does the gallery only ever create a single slider and a single set of crop controls?  If they get recreated, I think you'd better call stopDetecting() in the apprpriate place.