The reason you are getting that error is because the session no longer exists. It has gone away after you have checked out.
In order to get the info back you will need to first store it in your db, then after the transaction you can repopulate the cart with this info.
To get the info into the db to begin with you will need to ensure that you have the store order summary and store order details server behaviors on your confirm page before the user leaves and checks out.
Once you can confirm that you have the order summary and details stored in the db you can begin with the next part. This next part is going to be to create a recordset that is filtered by the order id or order reference id, whichever value you get back from your gateway.
Do you know what values you are getting back from the gateway? You must ensure that you are getting back some identifier so that the order can be identified within your db. With this recordset you will be able to repopulate the cart. You could then show the user a read only cart display on the checkout success page that will show them the items purchased.
You can also send out an email receipt on this page by using Universal Email. You will just need to add in the Universal Email server behavior to the page. Please post back with any questions that you have about any part of this and I will be glad to help you get this all going.