Gap between stand up shower and stud
I would cut out the drywall to the highest open point (left wall in your photo) all the way around (or at least along the back wall). Cut tapered shims for each stud from the necessary thickness at the bottom to zero at the top. Rest them above the shower flange, and cut additional (thinner) shims for behind the flange. 

You want the tapered shims flush with the outer face of the flange at the bottom. This will virtually eliminate any visible oddity in the drywall.