How do I set all ungraded items to zero in Gradebook?
Note: Gradebook does not include ungraded items in the course grade calculation. This could result in a higher course grade average for a student if he or she has incomplete items. To include a Gradebook item in the course grade calculation, you must enter a grade of zero or higher. If you want to enter zeros for all Gradebook items that have not been graded, you should set ungraded items to zero. This can be done for all students and all Gradebook items in a single step.
Alternately, to update missing grades for a single Gradebook item refer to, How do I set empty cells to zero for a single Gradebook item?
Warning: If your gradebook contains one or more items pertinent to only a subset of the entire class, using the function below for setting a zero score for empty cells will adversely affect scores for students not included in those items. For more details and guidance, refer to the article, How should I handle gradebook items not pertinent to the entire class?
Go to Gradebook.

Select the Gradebook tool from the Tool Menu of your site.
Click Update.

You will be prompted to confirm this action. If you are sure you want to assign zeros to all ungraded items, click Update.
Note: You cannot undo this action, so be sure you are ready to assign zeros before continuing!