Ryan Cartwright has written an article on how to backup Gmail with Getmail. It is an excellent piece, but Getmail is a command line program, and while most readers of FSM will doubtless be at home there and in configuring XML files, we also like to behave like electric currents and take the course of least resistance to get from A to B. Besides, using the Thunderbird (or Evolution) e-mail clients to backup Gmail has other advantages too. Let me explain.
To the chagrin of their competitors, GoogleMail seems to have become almost as synonymous with webmail as Google has with search engine (recently my six year old was explaining to me how he Googled for something at school). GoogleMail is a useful tool and has a lot of advantages over traditional client-server mail accounts, particularly if you are on the move. To be honest those sorts of advantages are present in pretty much any webmail setup: I'm just concentrating on GoogleMail because it's by my experience the most popular. But GoogleMail has one disadvantage, all your messages are stored on Google's servers. If you lose access to Google service or to your account then you lose your e-mails. Fear not oh free software lover, help is at hand in the form of the very useful getmail.