Not exactly a part of the GNU (or Linux) project, but Sylpheed works with Gmail over IMAP.
Formerly known as Sylpheed-Claws, it [Claws] started in April 2001 as the development version of Sylpheed, where new features could be tested and debugged, but evolved enough to now be a completely separate program. It forked completely from Sylpheed in August 2005.