Just to make sure I understand: the issue is that gmail may produce flags with [] in them, and imaplib currently fails to process such flags when it receives them from gmail?  

In principle I think we would not want to allow imaplib to be used to create such flags unless the user specifies some sort of "I want to violate the RFC" flag (which they might want to do, for example, to run tests against gmail :)  But currently it looks like it can?  (I haven't looked at this in enough detail to be sure.)  If that's true we probably have to continue to allow it for backward compatibility reasons, but we should document the RFC violation and possible consequences (an IMAP server rejecting such flags).