You can unlink a gradebook from a class in Google Classroom without losing the assignment columns and scores added through Google Classroom; however, once unlinked, the scores in those columns lock until you link the gradebook and the original class in Google Classroom again.
To unlink your TeacherPlus Gradebook from a class in Google Classroom, do the following:
In the Score Column Header, to the right of the Gradebook's class name, click
.

- In the Unlink Google Classroom dialog box, click Unlink, and then, in the Confirmation dialog box, click Yes.