You would need to hand code the funciton to pass the payment to the gateway using the documentation provided by that gateway.
We can help you implement this through as premiere support appointment:
http://www.webassist.com/premier_request.php
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.