In case anyone else has this problem: when I switched my gmail account to OAuth2, it brought up a sign-in screen which said “powered by Chrome.” I never could get it to let me log in…it kept wanting me to add the account through Android Accounts.
Finally I found an option in the menu to “open in browser,” and I opened it in Firefox. At that point it was simple, just a straight log in, and it seems to be working fine now.
Thanks for the OAuth2 support! Looking forward to Office365 OAuth2 support…I haven’t been able to access that account from K-9 for a couple of days now.