If I understand the question properly, [Aleksandr Blekh](https://www.quora.com/profile/Aleksandr-Blekh) answers the question well here: Are there any actual open source payment gateways, not just a library that integrates with other [payment gateways?](https://www.quora.com/Are-there-any-actual-open-source-payment-gateways-not-just-a-library-that-integrates-with-other-payment-gateways)
He mentioned three sources:
1. [Omnipay](https://github.com/omnipay/omnipay) 
2. [ActiveMerchant](https://github.com/Shopify/active_merchant) 
3. [CI Merchant](http://ci-merchant.org/), GitHub: [repo](https://github.com/expressodev/ci-merchant/)
Not entirely sure if this is what you are looking for

![download (3).png](https://steemitimages.com/DQmbhASRostPzWuwvY5WwJWCQmVJXAUmDuJvfsA16TpGLq8/download%20(3).png)