To run in Debug mode you could set the environment variable GAUGE_DEBUG_OPTS to the port number you want to attach a debugger to the process.
Gauge-java plugin will add the necessary JVM args required for running a debug server and wait for a debugger to attach before proceeding with execution, you do not need to set the gauge_jvm_args value.
For example, you can run gauge specs with the value GAUGE_DEBUG_OPTS=8001 and then run a Remote debug configuration to attach at port 8001 in eclipse.
This hasn't been documented yet.