Mymail-Crypt for Gmail is a browser (Chrome) extension that brings OpenPGP support to gmail. It aims to be a simple solution to mail encryption.
It is available in the chrome web store. This page also links to youtube which gives a quick overview on how to use the extension.
It is important to familiarize yourself with Options page, where keys will be loaded and other options set.
The project is built on top of JavaScript OpenPGP library OpenPGP.js. The extension uses the Chromium concept of Content Scripts to interact with the Gmail interface. It uses jQuery to manipulate the Gmail page.
I sometimes post about this project at prometheusx.net.
Additional help welcome. If the changes you're interested in making are in the OpenPGP component, please take a look at OpenPGP.js.
See LICENSE file for licensing info.