Actually, the new gmail is: https://protonmail.com/

Agreed. And: https://tutanota.com/

Gmail used to be good alright, but then it started to ask for my phone number all the time. I've switched to Tutanota recently, much better IMO, particularly the new client: https://mail.tutanota.com/

Is it open-source like protonmail?

Devs say they want to publish the app on F-Droid. https://tutanota.com/blog/posts/secure-mail-open-source