I'm not 100% sure I've got everything configured right, but I'm forwarding to Gmail from an Opalstack-managed mailbox and it's still hit or miss for me, even with SRS the last few months.
Some important messages (bank emails, one-time-passcodes), perhaps dependent on the sender's SPF settings, will always be marked as spam despite persistent training efforts on the Google side. I can whitelist ("Never mark as spam") the email using a Gmail filter but even then the Gmail app has a pretty annoying box saying to the effect of "hey, something fishy going on with this".
