Gmail confidential mode is not secure or private

Gmail’s confidential mode does not make emails private because Google can always read them. Other Gmail users can read the email in their inbox, but outside users only receive an email notifying them that a sender “has sent you an email via Gmail confidential mode” along with a link to a page on google.com. You can read a full explanation of how end-to-end encryption works)

ProtonMail also lets you send end-to-end encrypted emails to non-ProtonMail accounts (such as your friends and family on Gmail, to prevent Google from reading your messages to them).

Source: protonmail.com