Another approach is to set your “display gable” to Wall and select an appropriate wall type in the drop-down that becomes available below it. You can insert windows etc into gables using this method.
The issue with this method is that “display gable” walls are inserted “on center” so you have to adjust your roof insertion line to the mid-point of the wall below, instead of exterior edge in order for the gable wall to align vertically with the wall below, otherwise it will hang over. Also, add that half-wall thickness to your gable’s overhang to set it in the correct position.
I probably use the method FynrDzynr suggested more often though, depends on the situation.