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If I were you, instead of trying to do this directly in RVIZ of Gazebo I would look at how to process the costmap in a custom node, and from there extract the information you want.

Maps in ROS are represented as Occupancy Grids. This stores a discretized (i.e. cells) representation of the environment. From this representation, with the correct logic in a roscpp or rospy node, you can extract the information you want. Here is an example in the ROS nav stack Global Planner Costmap which manipulates this discretized representation in c++. Costmap2D is not exactly a occupancy grid but the concept/representation is similar.

Once you have the grid available to you in c++, I imagine you can produce the logic required to generate the result you want.