I have been evaluating different PGP applications trying to pick the best PGP desktop software. I use Gmail on both Windows and OS X, so I want something cross platform and free.
I’ve tried both the open source Gpg4win package for GnuPG and the commercial PGP Desktop. In my experience, the open-source applications I tried were too buggy, incomplete, and unfriendly to be worth it, especially to the non-technical user. By contrast, if you are willing to pay $99, PGP Desktop is much easier. For occasional use, the freeware mode (tutorial) of PGP Desktop works just fine. I did find a GnuPGP tutorial for OS X, but my experience with the Windows front-ends has discouraged me from trying it.
I also tried FireGPG, a Firefox extension that integrates with Gmail. FireGPG still requires GnuPG (and must be reinstalled if you don’t install that first!) but it seems to be the simplest cross-platform PGP + Gmail solution. FireGPG works well enough, although the whole process may still be too difficult for the average user, and the buginess of the GnuPG suite let me to stick with PGP Desktop.
Until something radically easier comes along, I’m going to continue recommending the free or paid version of PGP Desktop for the average user.
I’ve used Enigmail (http://enigmail.mozdev.org/), which is an extension for Mozilla Thunderbird (and Seamonkey), and it seemed relatively simple to use. The only way this will work with Gmail is if you setup Thunderbird to use Gmail’s SMTP and IMAP (or POP3) servers, and then you would use Thunderbird instead of Gmail’s web interface. I think it requires GnuPG to be installed, but I didn’t have to do anything directly with GnuPG (I’m using Ubuntu Linux). This is a little technical, but I think reachable for the average user, if they follow the setup instructions on Enigmail’s website, and the server configuration information on Gmail’s website.
When I was using Enigmail, it was when I was hosting my own mail server and using Thunderbird regularly. My domain name has since expired, and I now use Gmail for email and have put some thought into possibly using my mail server software to automatically download the messages from Gmail and relay my sent emails through Gmail, and once again use Thunderbird with this setup. I will then be able to get the benefits of using Gmail normally, while also having all of my email stored locally, which can speed up access to my email as well as provide a good backup (in the rare case that something might go wrong at Gmail) and provide access to any potentially important emails, even if my internet connection goes out for a period of time (which might happen a few times a year, but usually isn’t a big deal). This is beyond what the average user would do, however.