Hello, you should have a laser link which represents the origin of your laser hardware attached to base_link or etc.. in your model such as in turtlebot 's urdf
<joint name="laser_joint" type="fixed">
<origin xyz="-0.065 0 0.075" rpy="0 0 0" />
<parent link="base_link" />
<child link="laser" />
</joint>
<link name="laser">
<visual>
<geometry>
<box size="0.02 0.035 0.002"/>
</geometry>
<material name="Green"/>
</visual>
<inertial>
<mass value="0.001"/>
<origin xyz="0 0 0"/>
<inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.0001" iyz="0.0" izz="0.0001"/>
</inertial>
</link>
then in order to use this laser scanner in gazebo you should add laser scanner plugin
<xacro:macro name="turtlebot_sim_laser">
<gazebo reference="laser">
<sensor:ray name="laser">
<rayCount>180</rayCount>
<rangeCount>180</rangeCount>
<laserCount>1</laserCount>
<origin>0.0 0.0 0.0</origin>
<displayRays>false</displayRays>
<minAngle>-130</minAngle>
<maxAngle>130</maxAngle>
<minRange>0.08</minRange>
<maxRange>10.0</maxRange>
<resRange>0.01</resRange>
<updateRate>20</updateRate>
<controller:gazebo_ros_laser name="gazebo_ros_laser_controller" plugin="libgazebo_ros_laser.so">
<gaussianNoise>0.005</gaussianNoise>
<alwaysOn>true</alwaysOn>
<updateRate>20</updateRate>
<topicName>scan</topicName>
<frameName>laser</frameName>
<interface:laser name="gazebo_ros_laser_iface" />
</controller:gazebo_ros_laser>
</sensor:ray>
</gazebo>
</xacro:macro>