As reported in Lifehacker, Gmail is now providing an unsubscribe option for mailing lists to users. They are doing this using the mailto: type of the List-Unsubscribe header. This means that you can get a complaint message in ARF format when someone unsubscribes from you at Gmail.
How do you take advantage? According to the Gmail postmaster:
1) You authenticate your mail with SPF and DKIM
2) You have a good sender reputation
3) You have a mailto: list-unsubscribe header.
If you do this and a Gmail user chooses the “unsubscribe and report spam” button shown in the Lifehacker post, you should magically get a complaint message to that mailto: link, which you should immediately remove from your mailing list.
Gmail doesn’t have a feedback loop that is open to most mailers, but here is a way to get some idea of what Gmail users are complaining about.