Grant,
It occurs to me that you may not have to use GDL.
If you make your gable fascia with a roof with zero pitch
in the floor plan you can then make the mitered ends
by editing the edge angles of the roof.
I am imagining a parallelogram shaped form with one edge
having a chamfer.
View the shape in 3D the way you want the gable fascia object
to look if looking down at it and
"save contents of 3D window as GDL object"
Your gable fascia object will have the mitered corner to join
with the horizontal fascia.
I think this might work.
Peter Devlin
P.S. see this thread about making objects from the 3D window
http://archicad-talk.graphisoft.com/viewtopic.php?t=5201&postdays=0&postorder=desc&&start=10&sid=65b...