GnuPG is a crypto framework that can be used by other applications. This page lists some Free Software email clients you may use.
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)
Until Version 2.2.6 Gpg4Win came with Claws Mail; an email-Client that supports GnuPG crypto natively. Since then Claws is available as a separate package to ease maintenance of both packages. (The Gpg4win Initiative originally created the windows installer of Claws many years ago and the Claws Initiative now maintains it.)
You can find the latest release on the Claws Mail homepage.
(Should be build with and enabled gpgme support to have the first grade GnuPG integration.)