Here would be the place to look. It is part of the gazebo_ros_pkgs which you can either apt-get the debian, or download the source for github and compile it yourself. Many gazebo plugin's are listed here, and what you are look for (if your drivetrain system is what i think it is) is the SkidSteerDrive plugin which you will find towards the bottom of the page.
<gazebo>
<plugin name="skid_steer_drive_controller" filename="libgazebo_ros_skid_steer_drive.so">
<updateRate>100.0</updateRate>
<robotNamespace>/</robotNamespace>
<leftFrontJoint>front_left_wheel_joint</leftFrontJoint>
<rightFrontJoint>front_right_wheel_joint</rightFrontJoint>
<leftRearJoint>back_left_wheel_joint</leftRearJoint>
<rightRearJoint>back_right_wheel_joint</rightRearJoint>
<wheelSeparation>0.4</wheelSeparation>
<wheelDiameter>0.215</wheelDiameter>
<robotBaseFrame>base_link</robotBaseFrame>
<torque>20</torque>
<topicName>cmd_vel</topicName>
<broadcastTF>false</broadcastTF>
</plugin>
</gazebo>
Just keep in mind, this format is for a URDF formatted file. If you are using SDF, You should just take out the gazebo tags.