Mymail-Crypt is a Chrome browser addon to encrypt messages with GPG operating within Gmail webmail interface, the project aims to be OpenPGP compatible to be able to communicate with anybody using public key encryption even if they have different PGP or GPG software. After installing Mymail-Crypt you will have to generate your encryption keys, this can be done with the addon, entering a password is optional and highly recommended, if you don’t use a password anyone breaking into your Gmail account will be able to decrypt sign and encrypt messages supplanting your identity. Encryption keys can and must be backed up.
Mymail-Crypt is fairly easy to use, you will see a button in Gmail compose screen with the options “Encrypt and sign“, “Encrypt“, “Sign“. Received encrypted Gmail messages can be read using the drop down menu “Decrypt” option and entering your password.
Another way to encrypt Gmail messages with GPG is using Thunderbird and Enigmail but it won’t work for webmail, or obtaining a digital certificate for your email client.