Looks like this was solved in branch develop at Jan, 6, 2019:
98355b2d ( tekka 2019-01-06 18:14:10 +0100 158) delay(1000);

@mfalkvidd The development branch has the same behavior. An edit in core/MyGatewayTransportMQTTClient.cpp (I added a "delay(3000)" after line 148 in reconnectMQTT(void)) made it a lot better!

mysgw (the mysensors gateway) sends a continuous flow of error messages to the log file if mysgw is started and the configured mqtt broker is not available. I am running a quite recent version (built on Dec 5, 2018), branch master.
My problem with mysgw is the excessive logging. I know I can resolve it by configuring my mqtt broker, but mysgw should not produce such a massive amount of logging if the mqtt broker is not available.

Dec  5 11:19:13 raspberrypi mysgw: failed to connect
Dec  5 11:19:13 raspberrypi mysgw: GWT:RMQ:MQTT RECONNECT
Dec  5 11:19:13 raspberrypi mysgw: connect: Connection refused
Dec  5 11:19:13 raspberrypi mysgw: failed to connect
Dec  5 11:19:13 raspberrypi mysgw: GWT:RMQ:MQTT RECONNECT
Dec  5 11:19:13 raspberrypi mysgw: connect: Connection refused
Dec  5 11:19:13 raspberrypi mysgw: failed to connect
Dec  5 11:19:13 raspberrypi mysgw: GWT:RMQ:MQTT RECONNECT
Dec  5 11:19:13 raspberrypi mysgw: connect: Connection refused
Dec  5 11:19:13 raspberrypi mysgw: failed to connect

Started on the command line (only first lines copied, there are hundreds of lines per second):

root@raspberrypi:/home/pi# mysgw
Dec 05 11:26:42 INFO  Starting gateway...
Dec 05 11:26:42 INFO  Protocol version - 2.3.0
Dec 05 11:26:42 DEBUG MCO:BGN:INIT GW,CP=RPNGL---,VER=2.3.0
Dec 05 11:26:42 DEBUG TSF:LRT:OK
Dec 05 11:26:42 DEBUG TSM:INIT
Dec 05 11:26:42 DEBUG TSF:WUR:MS=0
Dec 05 11:26:42 DEBUG TSM:INIT:TSP OK
Dec 05 11:26:42 DEBUG TSM:INIT:GW MODE
Dec 05 11:26:42 DEBUG TSM:READY:ID=0,PAR=0,DIS=0
Dec 05 11:26:42 DEBUG MCO:REG:NOT NEEDED
Dec 05 11:26:42 DEBUG MCO:BGN:STP
Dec 05 11:26:42 DEBUG MCO:BGN:INIT OK,TSP=1
Dec 05 11:26:42 DEBUG GWT:RMQ:MQTT RECONNECT
Dec 05 11:26:42 ERROR connect: Connection refused
Dec 05 11:26:42 ERROR failed to connect
Dec 05 11:26:42 DEBUG GWT:RMQ:MQTT RECONNECT
Dec 05 11:26:42 ERROR connect: Connection refused
Dec 05 11:26:42 ERROR failed to connect
Dec 05 11:26:42 DEBUG GWT:RMQ:MQTT RECONNECT
Dec 05 11:26:42 ERROR connect: Connection refused
Dec 05 11:26:42 ERROR failed to connect
Dec 05 11:26:42 DEBUG GWT:RMQ:MQTT RECONNECT
Dec 05 11:26:42 ERROR connect: Connection refused
Dec 05 11:26:42 ERROR failed to connect
Dec 05 11:26:42 DEBUG GWT:RMQ:MQTT RECONNECT
Dec 05 11:26:42 ERROR connect: Connection refused
Dec 05 11:26:42 ERROR failed to connect
Dec 05 11:26:42 DEBUG GWT:RMQ:MQTT RECONNECT
Dec 05 11:26:42 ERROR connect: Connection refused
Dec 05 11:26:42 ERROR failed to connect

@scalz A useful addition could be that the controller only runs on Windows. I bumped into this limitation after quite a few succesful steps. Is there a Linux or Java alternative?