Uh oh, I think I have one of these. I somehow have my own custom domain pointing to gmail, for free, that I don't actually remember how I even got it, I've had

Uh oh, I think I have one of these. I somehow have my own custom domain pointing to gmail, for free, that I don't actually remember how I even got it, I've had it for years, but never have paid for it. That's all I do with this... whatever it is, email on a custom domain, I don't use anything else. I didn't receive any email this morning notifying me about it though. I guess maybe it's finally time to move to fastmail. I'm not giving up a custom domain email address. I don't like that I'll lose access to giant archive of received email. Is there a way to export all my archive/inbox from gmail? Import into fastmail? It looks like... maybe. But the fact that I'm trying to keep the same domain is going to complicate things maybe, since I can't easily have them both active at once to import from one to the other.