I think this object is not a GDL boject from the ArchiCAD Library. Rather, it was imported from a 3DS, DWG or SketchUp file, hence the triangulation.
You could convert the object to a Morph, set the edges you don't want to see to Hidden for the Morph, then resave it as a GDL Object. That could get you closer to the state you want it to be in.
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