If you own your own domain name for your email, then it's pretty simple and will take a few hours of effort to get fully moved over.

Part of why I pushed myself to move away from Gmail was I was still using the @gmail address occasionally.

Luckily, I'd been using my own domains for a long time for most emails, so it's been a pretty easy transition - I still have Fastmail poll Gmail for emails to make sure I see the ones that still go to @gmail.