Do you mean to access Fastmail as a client for your Gmail-hosted inbox? How about using the fetch feature that moves all emails to your Fastmail inbox, and the 'send as' feature where you can send emails out from your Gmail SMTP via Fastmail. Those two features already exist.
Although when setting this up, Gmail really make you feel like they might pull support at any minute.
I wonder if Fastmail could log you in to Gmail in a way that's consistent with Google's security model, similar to how you can "log in with Google" on many services. I'd much prefer it over the app password thing.
Now that you mention it, I remember that's how Fastmail works today. They use app passwords for non-Google-hosted email inboxes, and 'login with Google' for Google-hosted inboxes.
