Hi!
If your plugin is created as regular payment gateway, which supports WooCommerce Subscriptions you are good to go with our Flexible Subscriptions. But currently there’s no way for our plugin to support remote subscriptions, which will be handled on gateways’ end and only communicating status updates to Flexible Subscriptions.
Although it is not officially supported, you are welcome to test such workflow and report any findings. We would be glad to improve our plugin and support remote subscriptions. As you definitely noticed, it’s still very young plugin and we still gaining experience with more complex features. Let’s make it better for both of us!