Please switch to gmime 3.0
GMime 3.0 was released back in March 2017 and offers a bunch of improvements, especially for GPG handling.
See the GMime 3.0 porting guide.
The Debian gmime maintainer would like to remove 2.6 from Debian by the end of the year. Sorry this bug wasn't filed sooner. None of the people maintaining balsa in Debian actually use it.