how to see the robot path

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If you can use ROS/rviz (and do not have to do it in gazebo), this Q/A on ROS Answers might provide a solution for you.

There's also ticket on Bitbucket on rendering via messages. I'm not aware of any other possibilities to do what you require, but I hope someone else will chime in in case I missed them.

/edit: In reaction to your comment: If your want to compare data, it might make sense to also visualize the ground truth trajectory coming from gazebo in rviz. You could do this by using the Gazebo ROS API to retrieve the ground truth model state. Another option would be to add a gazebo_ros_p3d plugin similar to how it's done here to your robot to output ground truth data to a topic. Both method would enable visualization of the ground truth pose from gazebo in rviz. If you really want to render directly in gazebo, you will have to wait for the issue on Bitbucket to be resolved. Luckily, Nate just marked it critical ;)