Synchrophasor technology has numerous applications ranging from simple grid monitoring/visualization to real-time protection and control. Most legacy phasor measurement units (PMUs) and phasor data concentrators (PDCs) deployed in power grids support the IEEE C37.118.2 communication framework, which is highly vulnerable to cyber attacks due to lack of inherent security mechanisms. The IEC 61850-90-5 recently emerged as new communication framework with support for security features but its use in commercial devices is still very limited. The replacement of legacy PMUs/PDCs in power grids is a big challenge due to cost and deployment complexity. The concept of a gateway has recently been proposed in the literature to enable IEEE C37.118.2 compatible PMUs to send data in IEC 61850-90-5 format. However, the published gateway has limited features and also lacks security functionalities. This paper addresses security, interoperability, and integration issues between legacy and state-of-the-art phasor devices through the design of a security gateway. The security gateway is implemented with flexibility in mind and can be used for PMUs as well as PDCs under different configurations. It provides: 1) protocol conversion functionalities (from IEEE C37.118.2 to IEC 61850-90-5 and vice versa) and 2) security functionalities based on IEC recommended group domain of interpretation security mechanism. The security gateway is very compact in size, based on low-power ARM processor and inexpensive to be deployed in power systems. Through detailed experimental evaluation with real PMU data, this paper also validated the suitability of the security gateway for different types of synchrophasor applications with strict latency and data rate requirements.