I think there should be an option to export csv assignment grade files from right within the Gradebook, in this flyout menu:
Thank you for posting in the Community!
I understand that you prefer and encourage the idea of the feature to be added. We have submitted a feature request to the product team to add an option to export the assignment grade files into a CSV file.
We appreciate and encourage you participating in Community in making our products better. Thank you again for your input and feel free to reach us with any suggestions for improvement for our products