The Management Gateway Service (MGS) is the unimaginatively named gateway between the control plane and entities on the management network. In particular, it passes messages from the control plane to service processors (SPs) within sleds, switches, and power shelf controllers, and it routes alert messages from those entities to the control plane. It is used for inventory inspection both of the rack and its subcomponents. It allows for power management both via SPs and the lower-level Ignition subsystem. It is the mechanism for updating components. And it is the service for routing errors generated from SPs and Ignition.
For further details see RFD 210 which describes this service, the management network, SPs, etc.