FireGPG is a Firefox extension under GPL which brings an interface to encrypt, decrypt, sign or verify the signature of a text in any web page using GnuPG.
Its about time.
If you use this with Gmail, no matter what happens, no one can read your email on the server. Not that Google will do that of course.
Quite why Google didnt do this themselves is a good question. They have the expertise, and the understanding to do it. Certainly the Bad Guys® would discourage them from releasing a tool like this to all Gmail users; it would mean email messages going dark to ECHELON, and then the sky falling.