The gateway logs to stdout on the containers since usually a dockerized command is not run as a daemon, you can just tail the docker container logs to see what is going on in real time:
sudo docker ps
sudo docker logs --tail=100 --follow tyk_gateway_1
Obviously replace the gateway container name with the one for your system.
In Ubuntu, if daemonized, logs are under
var/log/upstart, on RHEL, if daemonized, under
This error is a connection error, usually something like "connection refused" or "no route to host", normally down on the tcp layer. The gateway output will tell you more.