Zelle is for person-to-person payments. They do not have a merchant API. So a payment gateway for it would be impossible.
Additionally at this time we are not planning to implement any further payment providers. Stripe and PayPal are high quality payment providers that cover 99% cases, full backwards compatibility, and top notch support. I honestly don't recommend using any other provider. More providers may come once we've switched over our payment API to something like OmniPay so we don't have to exhaust hundreds of hours on gateway development, testing, and maintenance.