hi , anyone know how to create API for payment gateway?
Are you looking to create new payment extension?
Best way to do this is use any existing payment extension as an example.
This is after you review AbanteCart architecture and extensions API.
http://docs.abantecart.com/pages/developer/architecture.html
Remember to check Developer Tools extension to help you to get extension built.
http://www.abantecart.com/extension-developer-tools