The parallel co-simulation is not yet supported. i.e. with current pacer block, connecting to multiple Gazebo instance ( with different port number ) is not possible. Like, you can not define port number as variable in pacer block. Further, the preferences are stored in single port number, changing in port number in same MATLAB instance is not possible.
You can execute following commands,
>> numSims = 3;
>> modelname = 'pacerTest';
>> simIn(1:numSims) = Simulink.SimulationInput(modelname);
>> simOut = parsim(simIn)
But, as you are running these command in same Gazebo session, execute with parsim will not make any usage of parallel execution.
Further, to 'step' and 'pause' Gazebo, you can make use of following commands in MATLAB.
modename = 'pacerTest';