Hello,
Adding a pure gmail (@gmail.com) works very well. However if the domain is different (for corporate account, for example), K9 doesn’t figure this out and continues the normal flow.
I managed to login to the corp account from the sign-in flow, after entering a different (fake) gmail account (to trigger the auth flow), but at the end it still didn’t work, because apparently the fake email is used after the flow completes, then editing the mail continues with the normal flow. Ideally there should be a checkbox to force gmail accounts (oauth flow), or check DNS to see the domains are actually handled by google.