If there is a payment gateway that is not currently offered in the core Paid Memberships Pro plugin, see below for the general steps needed to create a new plugin.
This document outlines the technical steps for integrating a new gateway with Paid Memberships Pro. Our blog post here covers a few alternatives and some of the non-technical considerations to adding support for a new gateway.
It should be noted that adding support for a new gateway it is not an “easy” thing to do. Depending on the complexity of the gateway’s API and what features you need to support, adding a new gateway can get very costly. In particular, supporting subscriptions and recurring billing greatly increases the complexity of adding gateway support. We spend tens of thousands of dollars each year maintaining the gateways bundled with PMPro. Developers will likely charge a minimum of $5,000 USD to start a project like this.
If you do go about creating a plugin to add support for a new gateway, please feel free to get in touch with us. We will help as much as we can. We can answer specific questions about issues you are having (easiest if you can point us to a GitHub repo), and we would like to promote your work when you finish.