Using GPG in webmail allows email encryption and digital signing. GPG is a freeware tool based on the OpenPGP standard that is available for everyone.
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.
Direct links for Extensions
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.