That’s not what HSTS does. It asks the client to remember that you want to only use TLS for that domain and refuse to use unencrypted HTTP in the future.
11 karma · joined January 1, 2021
https://support.google.com/mail/answer/10957?hl=en
This doesn't stop Google from having your emails obviously, but it allows you to move over your more important accounts now, while handling the other accounts as they inevitably send you emails over time.