Online casinos can ensure compliance with the General Data Protection Regulation (GDPR) in payment processing by implementing robust data protection measures throughout their operations. This includes conducting thorough data audits to identify and minimize the personal data collected during payment transactions, obtaining explicit consent from players for the processing and storage of their information, and ensuring that they transparently communicate how the data will be used. Casinos should also utilize secure payment gateways and encryption techniques to protect sensitive information, regularly assess third-party payment processors for GDPR compliance, and establish clear data retention and deletion policies.