Add external payment gateway to Shopify

Hello, I need someone that is good in programing to do me a simple task. I need to add a payment gateway to my cbd webshop that is based on Shopify. The payment gateway im habibi a contract with is called PayU Biz Poland, and is not supported in the Shopify. So we need to add it manually by coding in the Shopify. I need help from someone who knows how to add an external payment gateway to Shopify.