My view is that one needs to look at the actual GDL and see how complicated it is, how it fits his purposes and which is easier and faster to do: modify it or do it from Scratch.
Generally I agree that many GS GDL objects are quite complicated, Doors/Window being to most complicated ones.
I checked the scripts of the Built-in Story Marker and it is not that complicated (For whom, of course, for many it already is). It does not call any macros. However, it has some parts that are written to provide the correct functionality.
The Marker Head portion is about 20% of its 2D Script and is not very complex, if one knows some GDL of course. So in this specific instance I would probably try to modify a copy of this deafult GDL object.
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