You just connect your Google account in the Fastmail settings and they set everything automatically

From what I can tell, it just uses standard protocols like IMAP (at least, it did when I moved from a non-gmail provider to it).

Gmail uses IMAP, but there are rate limits that seem to have been implemented recently: https://support.google.com/a/answer/1071518?hl=en

The rate limits won't stop someone from eventually migrating out, but it might take a while.

GDPR specifies the right to data-portability. In Europe services have to provide data in a machine readable, commonly used format for export.

(For just this reason.)

And that's why I think the GDPR is one of the best things the EU has ever done.

Thank god for GDPR. Shame on me for thinking that this kind of regulation was dumb

You're excused, based on the useless and annoying cookie notice law.

All the better to get out while you still can!