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The Gazebo ROS interface is designed to simulate the interface that the robot would see in the real world as closely as possible. The generic goal is that the robot software should not know the difference between the real world and the simulated world.
Because of that the position of all obstacles is not made available from the simulator. You can use tools like laser scanners to make estimates of the observable obstacles. This is one of the fundamental elements of robotics to build a model of the world around the robot to allow it to operate. There are many different approaches for different application areas. Some are more mature like 2D navigation or plain object recognition
If you really want all the obstacles with ground trute from the simulator you need to write a plugin and give it a ROS interface. To make that happen please read through some of the Gazebo Tutorials on that topic.