I suggest you look at NBSv3. It's easier to build gateways.
https://nbrightbuy.codeplex.com/releases/view/137507
http://www.nbrightproject.org/Documentation/Developerguide/PaymentGateway.aspx
If your stuck with v2 then there is a document here on how to create a payment gateway.
https://nbstore.codeplex.com/releases/view/34143
NOTE: This v2 code will only be kept for legacy site and will not be developed any more. All work is now going into NBSv3.
https://nbrightbuy.codeplex.com/releases/view/137507
http://www.nbrightproject.org/Documentation/Developerguide/PaymentGateway.aspx
If your stuck with v2 then there is a document here on how to create a payment gateway.
https://nbstore.codeplex.com/releases/view/34143
NOTE: This v2 code will only be kept for legacy site and will not be developed any more. All work is now going into NBSv3.