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>