until wppizza v3.x, no additional gateways – other than the ones already available (https://www.wp-pizza.com/gateways/) – will be added I am afraid. Once wppizza v3.x is released, there is definitely a plan to add more gateways in due course. However, which one will be available when I cannot tell you. In the meantime, there is a skeleton gateway in the plugins add-on directory you could use to develop your own if you wish (wppizza v2.x)