I'd like to subclass the existing GPE, so that it differentiates between GPE-Nation, USA, and GPE-City, New York. I see in the docs how to create new entity types, but not how to subclass what's already there. Can this be done, and if so, how? Thanks.
You cannot subclass an NER type. You have to train custom NER types for that. In my opinion, I would get the GPE entities, and then separate them into Nation and City based on a dictionary lookup. There are finite number of major cities and nations in the world, therefore a dictionary lookup would be more suitable here than creating a generalization.