How to creat gearbox joint in <gazebo> tag?

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My robot need gearbox joints,but I cann't creat it in urdf file,so I want to use <joint> tag of sdf file in the urdf file by <gazebo> tag,and I reference this tutorial Using a URDF in Gazebo.The following code is written in a xxx.gazebo file and is included by a xxx.xacro file.

    <gazebo>
    <joint name="joint_DownGear_LeftMainRocker" type="gearbox">
        <parent>DownGear</parent>
        <child>LeftMainRocker</child>
        <gearbox_ratio>1</gearbox_ratio>
        <gearbox_reference_body>body</gearbox_reference_body>
        <axis>
            <xyz>1 0 0</xyz>
        </axis>
        <axis2>
            <xyz>0 0 1</xyz>
        </axis2>
    </joint>
    <joint name="joint_DownGear_RightMainRocker" type="gearbox">
        <parent>DownGear</parent>
        <child>RightMainRocker</child>
        <gearbox_ratio>1</gearbox_ratio>
        <gearbox_reference_body>body</gearbox_reference_body>
        <axis>
            <xyz>1 0 0</xyz>
        </axis>
        <axis2>
            <xyz>0 0 1</xyz>
        </axis2>
    </joint>
</gazebo>

but it failed,there isn't the effect of gearbox in Gazebo simulation.So,there is anyone use gearbox joint like this.Please!Thanks!