This is a real challenge, but can be done without GDL as long as it doesn't need to be parametric.
It also seems torturous. It is, until you know it, then it is a two minute exercise.
Side mouldings:
Use the trace function to put your side view of the dormer on the floor plan.
Make the moulding section a custom profile.
Then make a Custom Profile wall - curved - as if on its side.
Take a side view in 3D of the moulding as if it was from the top.
Trim to roof taper using an SEO.
Mitre front end with a roof plane SEO
Save 3D view As a GDL object. Place the object
Across the front:
You could use the same section to make a wall and save that in a similar way using slab bits to SEO mitre the ends.
Dwight Atkinson