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.
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.