Advanced Concepts/examples
This engine house is a good example of adding a gable end.  All of the wall sections were cut the same height as shown in the basic clinic.  Then, the pitch of the roof and size of the clerestory was measured onto foam core and cut out.  Two were cut and each was hot glued on top of an end wall. The gable was glued on to the wall while it was laying on a flat surface.  You can see a "smear" of hot glue on the face of the wall along the horizontal joint where I  rubbed off the excess hot glue before it was completely cooled.    Its going to get covered, as long as its smooth it doesn't matter.
I find it more efficient to add the gable end than to try and cut the complete wall with the gable end.