There are several email clients out there, but only some support OpenPGP encryption natively. Since you install GnuPG into your system you are free of choice with your email client. This page just shows you some clients or plugins you can use. All programs listed here are Free Software, just like GnuPG itself.
Enigmail (GNU/Linux, Windows, Mac)
Enigmail is a plugin designed for Thunderbird. But it works fine with forks of Thunderbird, too. You can access it directly via Thunderbird (Extras->Addons->Search) or directly
You can find the latest release on Enigmail's homepage.
Outlook Privacy Plugin (Windows)
The Outlook Privacy Plugin is a plugin for Microsoft Outlook >2010. To use it, visit the homepage and follow the instructions.
You can find the latest release of Outlook Privacy Plugin on its homepage: http://dejavusecurity.github.io/OutlookPrivacyPlugin/
Claws Mail (GNU/Linux, Windows, Mac)
You can find the latest release on the Claws Mail homepage: http://www.claws-mail.org/