Installing/fixing ceiling drywall when trusses are uneven?
You'll have a header running across the 90° transition in the framing direction, so this shouldn't be that difficult to fix.  I'd remove the drywall from the last stud bay (shaded below)...



...and attach blocking to the header that will catch the sheetrock. Since you're getting air ingress there, I'd take the opportunity to insulate while you have it open. I'd also use fiberglass tape instead of paper along this joint when you put the drywall back up - I've found that it tends to hold up better along seams like this.