Digitalization in the industry brings opportunities for financial technology (fintech) to improve and automate financial services and transactions. Payment gateway is a type of fintech that provides payment services by integrating the users' business process with various banks and financial entities. The payment gateway requires handling different users' needs, and the implementation is also related to various payment channels and environments. This kind of variation in the payment gateway could be managed using software product line engineering (SPLE). SPLE is an approach to developing various products by defining commonality and variability to achieve higher quality, productivity, and time-to-market. The first process in SPLE is a domain requirement engineering that analyzes the variability of the payment gateway. This paper presents the domain requirement engineering process of the payment gateway. We observe the existing payment gateway to analyze the feature variation. The output is a variability model of the payment gateway system in a feature model. Based on the feature model, the reusable components can be designed to implement the payment gateway product line. © 2022 IEEE.