It's really simple to use GPG to encrypt and/or sign email you send through Gmail.

Generate your private key.

gpg --gen-key

Extract your public key, which you will send to people you need to communicate securely with.

gpg --armor --export marko.haapala@ihatespamaktagon.com > my_public_pgp_key

Install FireGPG and restart the browser. Use GPG in Gmail.