A possible bug/limitation:
If I export the Gradebook, it appears I cannot change a value in it and then simply re-import. I get CSV error feedback. This is for University of Michigan.
I have to delete some columns as a workaround. I deleted the Infrastructure column and Read-only columns.
It would be nice if exporting a Gradebook, making some edits to scores, and then re-importing works. The headings have IDs for columns. So, it seems that would be a good workflow to support.