Hello!
What version|tag|commit of MQTTSNGateway ( https://github.com/eclipse/paho.mqtt-sn.embedded-c ) do you use in Raspberry Pi image?
Do you use special way to build it?
( nRF5 SDK for Thread and Zigbee v1.0.0 http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.threadsdk.v0.11.0%2Fthread_border_router.html )
I've build MQTTSNGateway from source and it doesn't send GWINFO message in response to SEARCHGW.
I test two MQTTSNGateway on different host with appropriate config and get working only binary from Raspberry Pi image.
logs
20180607 115437.322 SEARCHGW <--- Client 01 01
20180607 115437.322 GWINFO ---> Clients 02 01