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The common way of doing what you're talking about is to add shapes and stuff for development in RViz, not Gazebo.

Gazebo is the physics simulator, you can think of it as the real world with a real robot in it. In Gazebo, you should only see what you would see if you were looking at your real robot (you wouldn't see it's center of gravity).

RViz is a visualization tool that is made for this kind of thing. In RViz, you can see any data that your robot "sees" or knows about. Gazebo outputs information about the robot's sensors (i.e. joint angles) like a real robot would, which allows you visualize how robot thinks it is posed in RViz (by viewing the RobotModel in RViz). Then you can add things like center of gravity, velocity vectors, etc as lines, spheres, etc (these are called markers). Here are some tutorials on how to do that last part.