The Gmail Connection feature is very nice--when I receive an email in Gmail, it immediately shows up in Proton Mail. However, it continues to remain in Gmail until I go back and delete it. This means I have an ever-growing collection of unread emails in Gmail. In my opinion, this is a Proton Mail bug because it will eventually cause Gmail to hit its storage limit and subsequently for new Gmail messages to not appear in Proton Mail.

There is a similar feedback issue requesting an enforced retention period for synced mail, but I think there is a much simpler and intuitive solution: When I permanently delete a Gmail-synced email in Proton Mail, then permanently delete the email in Gmail.