Got Your Back (GYB) is a command line tool for backing up your Gmail messages to your local computer. It uses Gmail's API over HTTPS.
You can download the current GYB release from the GitHub Releases page.
The GYB documentation is hosted in the GitHub Wiki
The GYB mailing list / discussion group is hosted on Google Groups. You can join the list and interact via email, or just post from the web itself.
GYB is maintained by Jay Lee.