Drywallers messed up tile backer board, can I still tile?
That big of a gap is a problem, especially the gap in the 2nd picture. It is standard practice to ensure that the waterproof membrane overlaps the vertical lip of the tub at the wall, so that any moisture that penetrates through the grout (yes, grout is porous whether or not you use grout sealer) will make its way down into the tub well rather than into the wall/stud bays.

This is a quote from the DensShield tech. sheet: