How does the drywall meet a direct-to-tub surround edge?
It goes over the lip, but you need to add some shims (vertically along each stud),  to transition the drywall out, so it doesn't break when screwed down.

A bit of J-channel (over the lower edge of the drywall) makes a nice clean edge.  

Leave the drywall 1/4 - 5/16 above the surround edge and fill with a waterproof caulk.