Pose and orientation of my robot is incorrect in Gazebo and RViz

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There are two things that I think might be the issue, number 1, when you activate physics in Gazebo it falls aside because of its center of gravity or so, reason why you see it that way, number 2, you are using a launch file to call the model, and there you might be modifying the orientation of the model.

For solving number one (number two may vary), if you want the model to be fixed in the arena you can add the static attribute to the base of your model <static>true</static>, and also, to be 100% sure everything is ok, check if the base of your model is oriented the right way in the model file, this is the attribute you have to look for <pose>x y z roll pitch yaw</pose>.

x: Pose on the x-axis. y: Pose on the y-axis. z: Pose on the z-axis. Roll: Rotation on the x-axis. Pitch: Rotation on the y-axis. Yaw: Rotation on the z-axis.

Hope this helps!