What version of nopCommerce are you using ?
What external payment gateway are you using ?
Isnt there an existing payment method plugin for this payment gateway ?
What method are you using to post to the payment method ?
Look at how another payment plugin works - maybe paypal standard
I believe were are using version 4.0.
I am trying to use an internal gateway. It has been developed by another group and I am accessing it by posting a hidden form from one of our screens. I get to the gateway and process my payment fine, but when the gateway redirects back it goes to the login screen.
I will have a look at the PayPal plugin as you suggested.