I have the same question. I came up with a solution, but I’m not that happy with it and would love to know how to create separate grading periods in the class gradebook. Without this, the gradebook’s ability to calculate a student’s current grade is nonexistent past the end of the first grading period. So, my temporary and not entirely satisfactory solution: at the end of the term, I copy all grades to google sheets (of which I then make an extra copy, which I move to a different folder – just in case) AND I download all grades as CSV. Once I have these 3 copies of the gradebook (one in a google classroom folder, one in a separate folder in google drive, and one saved to my computer), I delete all assignments from the gradebook.
The downsides to this: I no longer have access to the work that students submitted for those assignments; I no longer have a record of what they turned in and when they turned it in; students lose access to the assignment posts.