According to the section "I lost my two-factor/mobile/etc (email/OTP|Authenticator/etc), what can I do?" of the GreenAddress FAQ page,
You should always have at least two two-factor options enabled.
Unfortunately, I did not. I only kept the Google Authenticator one enabled.
Additionally for OTP/Authenticator you should back up the initial seed number and/or QR code displayed. Provided you keep these backups you can maintain access to your coins even if you lose access to one method for any reason.
I did keep a backup of the Google Authenticator QR code (is this the same as the "initial seed number"?), but when I use the Google Authenticator or Authy apps to generate a OTP, GreenAddress / GreenBits doesn't recognize it.
Why would this be?
Is there a way to disable GreenAdress 2FA using the mnemonic and/or PIN?