Did you insert plugin on your urdf?

    <gazebo>
        <plugin name="p3d_base_controller" filename="libgazebo_ros_p3d.so">
        <alwaysOn>true</alwaysOn>
        <updateRate>10.0</updateRate>
        <bodyName>base_link</bodyName>
        <topicName>ground_truth/state</topicName>
        <gaussianNoise>0.0</gaussianNoise>
        <frameName>world</frameName>
        <xyzOffsets>0 0 0</xyzOffsets>
        <rpyOffsets>0 0 0</rpyOffsets>
        </plugin>
    </gazebo>