Given the existing Google Drive integration with the ability to link to Google Docs (although we'd like the ability to restrict this type of submission on a per assignment basis), I'm not clear why a student or teacher would need to upload a .gdoc file.

A .gdoc, .gsheet, etc. are just pointers to the actual files themselves in Google.  If you open these files in a text editor it reveals the content to be something like the following:

{"url": "https://docs.google.com/open?id=<unique Google ID #>", "doc_id": "<unique Google ID #>", "email": "yourusername@yourdomain.com"}

I don't see a need for this then.