I just copied the testmode.merchant.php file (plugins/wp-e-commerce/wpsc-merchants). Renamed it (in my case mynewgateway.merchant.php) and changed name, class_name, display_name and form in the first array.
Then changed the class name on line 29 (just changing test to mynewgateway, same as above), the name variable on line 31 and at last the form function name on line 42.
Works for me and my new gateway was added automatically in the gateway list on the Payments page.
You might want to do some more editing (comments and such) in the new PHP file though for your needs.