Over the last year or so, Google has added a lot of grading related features to Google Classroom to help teachers improve their workflow. Last school year, they added grading periods, scales, and calculations, and this past summer, they added "missing" assignments. Next up on their list is giving teachers the ability to "Grade All", which will copy one grade to all students. This feature is great when a teacher gives some sort of formative assessment that only has two grade options - like "done" or "not done" or 0 or 100. For students who did not complete the work, teachers could enter them as "missing" first. Then they would use the new "Grade All" feature to give everyone else whatever grade is associated with completing the assignment.
This feature was just announced and may not be visible to everyone yet, but it should be coming within the next few days or weeks if you don't see it now.
To use it, go to the Grades page in Google Classroom. Each assignment has a "⁝" (3 vertical dot) icon. When you click on it, you will see a new button that says "Grade All":
Then you can enter the grade. You also have the option to override existing grades (which you wouldn't want to do if you already put in the "missing" or other grades) and to return all grades automatically after you post them:
If you don't automatically return after grading, the grades will appear as drafts that can be returned later.
This new feature should save teachers lots of time if they use it over multiple assignments and multiple classes. Keep an eye out for this feature in your Google Classroom soon if you don't have it yet!