The only discussions I could find online about how to install ceiling drywall where it meets the gable end of a truss roof recommended installing nailers to make up for the lack of a truss at the end to which drywall could be attached, but I finally found a video from Prest-On, who recommend their Corner-Back (F-clips) or Framer-Back fasteners for attaching ceiling drywall to top plates to allow the drywall to flex when the framing moves, particularly due to truss uplift.  I find it strange that none of the dozens of articles and videos I’ve seen on installing drywall mention this solution other than a couple that mention it for interior partitions.  It seems much easier and more economical with a superior result that will reduce or eliminate cracks at corners.   Perhaps this will help the next person who searches for how to attach ceiling drywall at gable ends.  Here’s the video; the relevant part is from 5:00 to 5:18, roughly:
https://www.youtube.com/watch?v=-YqF1dfYVL0