On its 10th anniversary, Signal’s president wants to remind you that the world’s most secure communications platform is a nonprofit. It’s free. It doesn’t track you or serve you ads. It pays its engineers very well. And it’s a go-to app for hundreds of millions of people.
You set up the forwarding in google, not proton. You mark the forwarded emails in your proton mailbox. You forward the emails to your proton account until you changed all the sources that you care about from your google to your proton mailbox. Then you turn off forwarding.
Google never gets any more data from you except your protonmail address.