Gable openings are a known issue – currently the top of the opening has to be below the wall height on each end. Trying to resolve this and include in future release.
My current workaround has been to edit the gable wall and model the openings “manually” – you can use the standalone lumber tool to create the headers, etc. – you can copy and edit the adjacent studs and edit to fit. Not ideal, but works until we can get it automated.