GWMP is a GateWay Messaging Protocol used by LoRa packet forwarders to typically talk to a LoRaWAN Network Server (LNS).
On the Helium's LoRaWAN Network, the GWMP is used to send packets to a gateway client.
This program, gwmp-mux, allows for a single packet forwarder connection to be multiplexed out to one or more potential hosts. As such, a single gateway can connect to multiple LNSs, for example.