While it doesn't protect from Google's anti-privacy tactics, the proton Mail app does strip third party tracking from emails, which can be helpful.
Many companies use a single pixel transparent image (or similar) to identify whether a user has opened an email. They can then build a profile on which emails you opened vs left unread even if you didn't click anything.
Proton as an email frontend tries to prevent that kind of third party tracking. It also prevents Google from knowing things like how many times you open an email, how much time you look at it, etc.
It's not revolutionary, and it certainly doesn't remove the harms of staying with Gmail, but it's not pointless.
Reminds me of this:
https://youtu.be/JcJSW7Rprio