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.