Sheathing panels are 48 in wide, and each vertical joint has to land on framing. On a wall with a wide window, the nearest stud can be further away than a sheet. The straight-wall planner already handled that: it put the joint over the opening and called for a short nailer between the header and the top plate.
Gable end walls did not. They were laid out as if no opening could ever swallow a joint, so a wide window near the end of a gable wall came back as "no stud within 48 in of a panel edge; sheathe it by hand".
The gable planner now looks at the openings too. It places the joint over the opening, adds a note ("fit a 2x4 nailer about 24 in long on end between the header and the sloped top plate"), and the wall's self-check accepts that edge. The length runs from the top of the header to the underside of the sloped plate at the joint.
The nailer is a note, not a line in the lumber order. The test is a 20 ft gable end with a 10 ft window; it fails without the change. There is no real gable plan behind it yet, so treat the length as a starting point and check it on the wall.
The tool is the wall-framing takeoff; it is a planning aid, and the engineer or inspector decides the framing.