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You should probably read up on some basic concepts regarding rviz, gmapping and generally "how stuff in ROS works :)" (the tutorials are a good start).

Rviz is the ROS Visualizer and mainly is useful for visualizing the robot and environment state. It does neither build maps, nor does it provide algorithms for searching for objects in them. It just allows you to visualize the map it receives from somewhere else (for example gmapping).

That being said, I am not aware of a existing approach that performs some kind of object recognition on occupancy grid maps (if that is what you require).