How can I integrate NETCASH as a 3rd party payment gateway?

[BellaPratt]

Good day, 

We want to integrate a 3rd party payment gateway - NETCASH - into our Shopify store.

I received this link from NETCASH to guide me through the integration process but it is quite tedious as it does not only apply to Shopify - https://api.netcash.co.za/inbound-payments/pay-now/pay-now-ecommerce

Where do I start if I want to integrate a payment gateway that is not necessarily listed as an additional payment provider? I contacted Shopify support and they directed me to this page to pop the question on how to integrate a third-party custom payment gateway. 

Our NETCASH account is setup, and the 'Pay Now' is activated - I only need to integrate it with our online shop. 

Kind regards 

Bella