Yeah, I don't think there's any meaningful way to "port" a "gmail.com" email address away from Google. The entire internet infrastructure is set up so that can't happen, based on the meaning of "domain".

Seems like there's an opportunity there though. If someone could create a platform that would take your address, create you a custom domain, set it up, get your email flowing there (including porting over all your existing email out of gmail), and then helping you move your sign ins to that new address..

That'd be huge. It would also be very, very hard, the amount of infrastructure it would touch.. but doable.