This is most likely due to a recent change on Google's end; it might have removed support for old connection methods in third-party apps. Unfortunately, this isn't expected and as GoodNotes 4 no longer receives updates, there is little you can do except switching to GoodNotes 5 or using the following workarounds:
- For auto-backup, you can still use Dropbox, OneDrive, or Box as an alternative auto-backup location.
- To import files, tap the plus icon in the library view > Import > iCloud Drive. This will bring up the Files interface for you to choose files on Google Drive.
- Similarly, to export documents, please choose iCloud Drive as the target after tapping Export, and then choose a location on Google Drive.
If Google Drive isn't shown in the Files interface, please add it to the Files app.