This article details how to configure Gmail with PGP Desktop when using Mozilla Thunderbird as your POP email client.
To use your Gmail account with PGP Desktop, you must first enable POP for your Gmail account within your Gmail account mail settings.
To use a Gmail account with PGP Desktop, you must edit the port and SSL settings of your Gmail account in Mozilla Thunderbird and the PGP Messaging service in PGP Desktop. PGP Desktop is supported on Mozilla Thunderbird 2.0.
PGP Desktop Settings