So gradebook has an undocumented property to support entering letter grades. This was added way back in 2.5 in https://jira.sakaiproject.org/browse/SAK-9465
However the option to actually set this grade type was removed as part of
SAK-19964. I don't believe this was intentional, and adding it back in gets it all to work again.
Even though GB1 is deprecated, I think keeping this correct is probably a good idea, especially since GB2 supports it and if https://github.com/sakaiproject/sakai/issues/2730 is resolved.