There is a new update for GradeBook that improves import from Google Classroom. We have added a new item into the settings menu item (see image below):
 This is how it works: 
 Importing 
 You can choose whether you want Classroom assignment dates to be imported as due dates or creation dates. There is a tooltip that says that the due date will be substituted with creation date in case it's absent. This is where most people have issues, when some assignments have due dates and others do not. The sorting will first look for a due date and if no due date is present then GradeBook will use creation date. Sorting Sorting by date in GradeBook will use the values obtained upon import. We know it is not always possible however if all assignments had a due date or all had no due date, import would be much easier!
 To see if you have the latest version:
 https://docs.google.com/document/d/1_mj3Rsn-oRK7O-crvZ3kroGyZuoKBTThtEwbP6fYOUY/ 

