Maritime container gates involve a sequence of tasks, including vehicle and cargo identification, document verification, routing decisions, and access authorization. However, the information required to support these activities is often fragmented across multiple independent platforms. This paper introduces GateOS.ai, a Smart Gate information system designed for the Port of Sines. The study followed an echeloned Design Science Research methodology informed by stakeholder co-creation and human-centered design principles. The resulting artifact comprises validated checkpoint-based requirements, an operator-facing interface prototype, and a layered architecture grounded in Digital Twin concepts. The architecture connects gate infrastructure and captured events with data management, external systems, analytical models, and user decision support. Qualitative stakeholder reviews helped refine the proposed workflows and confirm their alignment with the intended gate process. The study provides a structured path from operational needs to system design and supports the gradual evolution of the gate environment toward a data-driven ecosystem.