The ability to link cloud storages like GoogleDrive, OneDrive and Dropbox is currently not supported in GoodNotes 5 and will be added back. We're sorry for the inconvenience. Please keep an eye on the App Store as we're frequently releasing new updates to GoodNotes 5.
Other unimplemented features that will be added back can be found in this article:
Update April 2019:
We've previously paused working on auto-backup and other missing GoodNotes 4 features in order to fix some high priority issues. We now resumed working on it and will try shipping it asap. Currently, there is no timeframe for it because we'll need to test the features internally before releasing it publicly.