Hi,
You seem to have a good grasp of it. The orderid of the store is usually past back to the store as a post field or url param in the autoresponse, so you can usually pick it up from there, using the Request passed into the event.
One problem you have with DNN6 is that it injects the full page content for the autoresponse, so you have to use a Notify Provider to pickup the autoresponse. This then doesn;t trigger the autoresponse event, unless the gaterway provider code in the notify provider tiggers it for you. So you must know what they are doing!
If your using multiple payment gateways, then you'll need to see how each one works....and see how they trigger those events.
I've uploaded the latest example source code for a gateway provider, you can find it here http://nbstore.codeplex.com/releases/view/34143 along with some documentation on gateway providers. (Unfortunatly I've not triggered the autoresponose event from the example code, so if the guy who wrote you gateway provider has followed my lead you'll not have the event triggered.......sorry an oversight on my part!)
Dave.