plugins are provided “as is”
I cannot provide support for the development of your own plugins/gateways (other than what exists in the add-ons directory for example)
please see FAQ’s
https://support.wp-pizza.com/topic/customisation/
PS: There is a skeleton-template in the add-ons directory of the plugin as a starting point to develop your own if you so wish. However, due to the many different ways gateways can be implemented, this is still a bit of a moving target, If you wish to develop your own based on those templates, make sure to test things thoroughly