There are many definitions of an IoT Gateway (Internet of Things Gateway) but two definitions which seem to capture the essence of the common understanding can be found in these web articles:
IoT Gateways provide a local (to the “Things”) point of computing resources to perform edge computing and data analytics and also serve as a focal point for communication connectivity aggregation for the “Things” to the Internet.
Interestingly enough, the original target device that inspired the formation of the OSGi Alliance in 1999 was the Gateway. Telecommunication operators and suppliers specifying and creating Gateway devices were among the strongest, most active participants in the Alliance back then, and many remain active Alliance members today.
The original technical requirements of the Gateway back in 1999 – compact, robust, adaptable, easily maintained, and dynamic that OSGi technology delivered upon, are just as valid for IoT Gateways today.
You can find an OSGi Alliance Member’s recent perspective on IoT Gateways in a webinar described in this OSGi Alliance blog article.