To enable custom message support, you need to use 'gazebogenmsg' in MATLAB. The .proto files or Gazebo msgs, you select while building custom message dependencies, only those topics you can see in Subscribe or Publish block.
There are few steps you should follow to see cutsom topics in Subscribe or Publish block.
1] You need to create custom .proto file or select Gazebo .proto message with 'GazeboMessageList'
2] Call 'gazebogenmsg' which generate required dependencies.
3] You need to follow steps mentioned at the end of 'gazebogenmsg' call
4] You need to package plugin including path of custom message dependencies with 'packageGazeboPlugin'
5] You need to copy and build GazeboPlugin.zip on VM
6] You can launch Gazebo .world
7] Further, on Simulink side, you can add Subscribe or Publish block to see those topics.