Publish a Gazebo message into a ROS topic?

edit

It is definitely possible to edit the plugin you already have. Another option is to use an already existing plugin in the gazebo_ros_pkgs library that publishes your laser data to a ROS topic. The plugin you are looking for is libgazebo_ros_laser.so. You can find the source code from here:

http://docs.ros.org/jade/api/gazebo_plugins/html/gazebo__ros__laser_8cpp_source.html

So you have two options here:

1) Modify your existing plugin to also have the functionality of advertising the laser scan data to a ROS topic by using the given source code as a reference.

2) Adding the existing gazebo_ros_laser plugin to your SDF file like so:

<sensor type="ray" name="sensor">
  <pose>0 0 -0.004645 1.5707 0 0</pose>
  <visualize>true</visualize>
  <update_rate>30</update_rate>
  <ray>
    <scan>
      <horizontal>
        <samples>32</samples>
        <resolution>1</resolution>
        <min_angle>-0.53529248</min_angle>
        <max_angle>0.18622663</max_angle>
      </horizontal>
    </scan>
    <range>
      <min>0.05</min>
      <max>70</max>
      <resolution>0.02</resolution>
    </range>
  </ray>
  <plugin name="sensor_plugin" filename="libgazebo_ros_laser.so">
    <topicName>/your/sensor/topic/name</topicName>
    <frameName>/your/frame/name</frameName>
  </plugin>
</sensor>

Notice the plugin tag at the end of the sensor tag. The libgazebo_ros_laser come inside the gazebo_ros_pkgs package which you probably already have installed.