[archenary]

I did this recently. It's pretty straightforward.

First, do a one-time import from Gmail. Fastmail has an import tool that does this over OAuth. Took me ~45 minutes to import ~50,000 emails.

Next, setup IMAP and SMTP on Fastmail for your Gmail account. This way, you can continue to receive and reply to emails sent to Gmail, using Fastmail as the client. When replying to an email, Fastmail defaults to the right sender (identity) based on whom the email is sent to ([e-mail] or [e-mail]).

An alternative is to setup email forwarding in Gmail, so you get a copy of emails sent to your old address.

If you don't have a custom domain, I highly recommend getting one and use that going forward. There might come a day when you want to migrate off Fastmail. With a custom domain, you just need to update the MX records.