Channeling Kyle Blythe:
If you have an existing comment in the GB, you cannot remove it by deleting it from the GB export .csv and re-importing.
1. Enter comment(s) into cells for a given GB item.
2. Export the GB as a .csv.
3. In the export .csv, delete the comment(s).
4. Attempt to import the file.
The "Comments" column will not register any updates, even though it should.