Apparently Unsubscribed MailChimp Contacts can not resubscribe via the MailChimp API (ie. via a GravityForm, see https://mailchimp.com/help/resubscribe-a-contact/#Resubscribe_with_the_Signup_Form). However, neither the form user nor the admin are notified that there has been an issue.
The GravityForm MailChimp Add-on Log states “was found on list. Status: unsubscribed” and “Unable to add/update subscriber: Member In Compliance State”. Since that information is available to GravityForms and the only way for a MailChimp subscriber to resubscribe is via the Mailchimp-hosted signup form, why not provide a convenient link for the user and a notification to the form admin?