The large-scale integration of distributed energy resources into modern power systems is still hindered by the coexistence of heterogeneous field devices and non-uniform communication interfaces. Batteries, inverter gateways, heat pumps, and other distributed assets often rely on legacy industrial protocols, which limits their direct integration into interoperable grid-oriented environments. This work presents DERway, a smart edge gateway developed to bridge this gap by providing bidirectional communication between legacy DER devices and standardised external services. DERway interfaces locally with field equipment through protocols such as Modbus TCP, Modbus RTU, and CAN, while exposing telemetry and control capabilities through interoperable interfaces including MQTT, REST, and IEEE 2030.5-oriented communication flows. The platform is structured around dedicated functional blocks for device acquisition, protocol adaptation, and interface management, enabling the translation of local register-based measurements and setpoints into externally accessible, semantically organised services. An internal topic-based communication layer supports the decoupling of device-specific communication from upper-layer services and simplifies the integration of multiple device types. The prototype was implemented on an edge hardware platform and experimentally validated in laboratory conditions with real DER-related assets and multi-protocol communication scenarios. The obtained results demonstrate successful device onboarding, telemetry acquisition, external publication of measurement points, and handling of write commands across heterogeneous interfaces. Experimental tests also confirm the feasibility of mapping device-level variables into standardised representations suitable for supervisory platforms and other grid-facing applications. The proposed gateway therefore provides a practical interoperability layer for legacy DER assets, reducing integration complexity and enabling their progressive inclusion in digitalised, renewable-rich energy systems. The work contributes with an implementation-oriented approach to DER integration, showing that edge gateways can play a key role in connecting heterogeneous field devices to modern power system communication frameworks.