The increasing complexity of space systems, coupled with their critical operational roles, demands a robust, scalable, and sustainable security framework. This paper presents a novel system-of-systems approach for the upcoming Lunar Gateway. We demonstrate the application of the secure-by-component approach to the two earliest deployed systems in the Gateway, emphasizing critical security controls both internally and for external communication and connections. Additionally, we present a phased approach for the integration of Canadarm3, addressing the unique security challenges that arise from both inter-system interactions and the arm's autonomous capabilities.