If GetResponse can support receiving PayPal IPN, it should be fairly trivial to adapt that to supporting our IPN-style custom/3rd-party Integration method:
http://www.e-junkie.com/ej/help.integration.htm
MailChimp already supports this method, as do several other third-party services listed on that help page; if you can support it, we would be glad to list and link to GetResponse as a recommended integration partner on that page as well.
At present this is the only integration path we can suggest, as our Development team is fully occupied with internal projects for the foreseeable future, implementing major system improvements and popular-demand new features and functionality. I see we've also replied to an email you sent us about this, so feel free to reply to that if you'd like to further discuss the matter more directly.