Sometimes there is not something out in the WordPress-o-sphere that does quite what you need and building something from scratch is the best way to go to make sure it does what you need. I ran into a need where I had to write a custom payment gateway for WooCommerce.  Came across this article and it was pretty much giving me a blueprint of what I needed to do.  It was very helpful to get a jump start in the work.

Building a WooCommerce Payment Extension

Building a WooCommerce payment extension is a straightforward process for the most part. The hardest problem that you'll probably encounter is learning how to interact with the merchant's API to submit a payment and parse the response back. In this article, we are going to build the real thing: a working WooCommerce payment extension.