If someone would write a plugin for Gmail to encrypt the message text it would be a huge seller, you could retire and live on your own island in the carribean.
How about an add-on to the right click menu of cut/copy/paste that would be “GPG Encrypt”? Select some text, right click and choose “GPG Encrypt” and hand it off to the Kleopatra API, then encrypt it, then paste the result in place of the selected text.
I would pay for that app!