GPGMail uses a very well known and white spread technology OpenPGP as its base. Everyone you want to use it with has to have a mail client which supports OpenPGP in one form or another, but there are many plugins out there, who add support for your favorite mail clients on windows and linux.

Enigmail for Thunderbird: http://www.enigmail.net/home/index.php

GPG4Win provide plugins for Outlook: http://gpg4win.org

Evolution on Linux has OpenPGP support built in.

FYI: Another term used for OpenPGP is gnupg or GPG, should you want to google for other solutions on other operating systems.