IoT sensors offer a wide range of sensing capabilities, many of which have the potential for health or medical applications. Existing solutions for IoT in healthcare have notable limitations, such as limited I/O protocols, limited cloud platform support, and limited extensibility. Therefore, the development of an open-source Internet of Medical Things (IoMT) gateway solution that addresses these limitations and provides reliability, broad applicability, and utility would be highly desirable. Combining a wide range of sensor data streams from IoT devices with ambulatory mHealth data would open the potential for providing a detailed 360-degree view of the relationship between patient physiology, behaviour, and environment. To harness this potential, RADAR-IoT has been developed as an open-source IoT gateway framework. Its purpose is to connect multiple IoT devices at the edge, perform on-device data analysis, and integrate with cloud-based mobile health platforms such as RADAR-base, enabling real-time data processing.