Your BLD file is a perfect example of how this is not working. The way you have done it is exactly what I do but in your case you have the wall set to AutoExtend so there is not a problem - that will work.
The problem is when you want to have a different gable texture, hence have the walls set to level and turn on the Display Gable.
As you can see in the image from your file both problems mentioned earlier are evident:
1. The Gable shoots out through the roof.
2. The fascia on this edge of the roof is not there. (It was not there before I turned the gable on either).
I did not manipulate your file in any way except the turn the Gable display on and give it a texture as it could not be seen against the white background (white for me - maybe I do not have your background image).
Try turning Display Gable om and see if you get the same result.
My example was just draw the walls, click in a roof (by perimeter), draw a line to align with the opposite roof, insert a break on the line and set that edge to Gable with the Display Gable ticked. Any missalignment of the roof with the walls has come about from me fiddling later.
Just in case, I switched to Imperial catalogue and UOM and did a new one, exactly the same result and that is with the roof and walls aligning using Roof by Perimeter. Unless I drag that gable edge of the roof right out, the gable fill will always go through the roof and the Fascia will vanish. This is the same in Imperial and Metric in Envisioneer7.2 and TFP16
I will wait until you finish dinner...