Dwayne:
Your issue is clearer now. You could set the angle of the edge of the slab, check it in a section to measure the horizontal distance from the top node to the displaced bottom node (or use algebra to calculate this distance), then use the pet palette (offset all edges) to expand the slab that distance to get the edge of the slab where you need it.
To answer your initial question, there is not a direct way to do this using GDL, you would need an API (beyond my pay grade) to read the fill coordinates into an object. There is an indirect method where you can drag a fill into the 2D Script of an object, then use the scripted coordinates as part of the code for the object, but this does not seem efficient for your purposes.
That's all I have. You could try posting some images of your problem, someone might see a better solution.
David