One problem with PGP, at least with GnuPG, is that it does not interact with the web. There used to be a Firefox addon for that, called FirePGP, but its development was stopped.
So, good news, a new addon has come to fill the gap it left: WebPG, an addons for Firefox and Chrome. I am using it since a while, and it seems to work fine, being able to encrypt, sign, decrypt and check text blocks. Of course, it cannot handle PGP/MIME unless explicitly adapted to the webmail you use, but there seem to be some experimental support for GMail.