In order to use end-to-end encryption, it is enough to use a browser extension - it uses open pgp - open source for encryption and keys and just inserts a button to gmail :)
https://youtu.be/aAXIqnjbc-M?t=106 Mymail-Crypt for Gmail at chrome store
quite radical, simple, but it can fall off :)
there is a more reliable way, but you have to manually encrypt it - another extension