7.3.4 Support for IMAP Extensions

If you’re using Google’s Gmail, you may want to see your Gmail labels when reading your mail. Gnus can give you this information if you ask for ‘X-GM-LABELS’ in the variable gnus-extra-headers. For example:

(setq gnus-extra-headers
      '(To Newsgroups X-GM-LABELS))

This will result in Gnus storing your labels in message header structures for later use. The content is always a parenthesized (possible empty) list.