Integrating a payment gateway into a mobile app involves several key steps to ensure smooth and secure transactions. First, choose a reliable payment gateway provider that supports mobile integration and complies with relevant security standards. Next, implement the gateway's SDK or API within the app, allowing seamless communication for processing payments. Ensure user data encryption and compliance with PCI DSS standards to protect sensitive information. Thorough testing across different devices and scenarios is crucial to guarantee functionality and reliability before deployment.