The lunar Gateway is a complex system-of-systems (SoS) requiring secure, resilient integration of all its systems. This paper develops controls for its early systems to secure command and data handling (C&DH) and evaluates their effectiveness through a case study on collision impact propagation originating from Canadarm3. We demonstrate that a baseline security strategy with stringent, standardized controls reduces failure impact by up to 70%, underscoring the importance of consistent security integration across all systems.