GPG is a freeware tool based on the OpenPGP standard that allows email encryption and digital signing.
To use GPG in webmail, you need to install a plugin/plug-in called Mailvelope in your browser. This is because Webmail can detect if an email is encrypted and display a message to that effect, but does not do more than that, key management and (de)encryption itself.
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Using Hardware Security Keys
In addition to logging in to Webmail, we recommend using hardware to store GPG/PGP keys.
For example, almost all YubiKey security keys have GPG support. If the key is stored on the YubiKey and otherwise works, Mailvelope may require a separate gpgme (or gpgme-json) installation.