Unfortunately, existing documents from Goodnotes 6 can not be automatically synced with your Classroom files. This is because Goodnotes 6 and Classroom utilize two different syncing services and file formats.
How can I move existing materials from Goodnotes 6 to Classroom?
It is still possible to manually move your existing content from Goodnotes 6 into your Classroom workspace. You must manually export and then import the documents as Lessons. You can find the instructions on how to move documents from one app to the other in this article.