The drywall attaches to the framing. So the framers frame the rough opening and then it gets covered with drywall.

If you want a very large, semi-elliptic arch like your inspiration pic, you should give the framers a full-size half-pattern to trace, because you will get something you may not like otherwise.
Casey

Our framers made patterns for the exterior arches (some brick, some stone). The interior arches were started by telling them how high we wanted the side walls to be, and we went from there by eye, to decide height of arch (center), etc