I'm not sure what you mean by "Payment Result" in your code behind.
Part of a Payment gateway is you need to define where the Payment Form Control (ascx) is defined, the system uses this control to allow the user to enter things like their Credit Card information or any information needed for the ProcessPayment method to function.
How it works though is you define a page in Kentico that is the "Payment Page" and include the webpart "Payment Form"
The Ecommerce, when sending a user to this page, sends the payment method to the Payment form which in turns loads the ASCX control for that payment method.
Part of the "Payment Form" Web part is the "Thank you Page Url" that a user is sent to after they are done, so this is separate from the Custom Payment Gateway itself. You can use this as the "Payment results" if you wish.
Does that make sense?