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Old question, but it came up in Google for me, so here it is-- an octomap is data structure for representing information about the environment. Gazebo is meant more as a physics simulator. For example, moving robots around a room with rigid obstacles such as a couch, table, and walls. The robot and obstacles are visualized as themselves, and they have configuration-file-defined physical properties. Some additional visualizations do exist, such as sensor field of view, but I see those as "extras". RViz, on the other hand, does not use a physics engine, and only displays data collected (in our case) from ROS topics. And that data has already been converted to some ROS/RViz-compliant representation, whether that's a point cloud, marker array, coordinate frame, etc.

tl;dr Gazebo is used for physical simulation, while RViz is used for data visualization. You'd likely use both at the same time in many instances. See this page for some examples.