Editing complex GDL is easier in MS Word, so just copy it into a DOC file, switch and edit anything you would like, then paste it back to GDL.
Relating to the subject - what if I parametrically defined all materials (switching 3D studio pens to "material xy" for complex meshes, yet of the 6 materials (in this specific case) it only works for 5, leaving one "undefined"? Is there any way to define all undefined as a certain material? Of course I woud prefer not to look through 3000 pages of GDL data to find the missing components.