Trouble with a part of a robot in Gazebo

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I am building a robot simulation with a UR arm and a Robotiq 85 2 finger gripper. I am able to load the arm and the gripper both attached in RViz without problems, I also can move the joints of both the arm and the gripper via joint_state_publisher. But problems appear when I try to load my system in Gazebo. The arm and the gripper wobble. I thought this might occur because the .yaml controller files are not very well formed because I am getting errors in the log. However, the thing that frightens me the most is that the base of the gripper is not appearing in the simulator. As I said before, it loads perfectly in Rviz. This seems to be a recurrent problem for many people using this gripper. But they have not been able to solve it neither. They've reached the solution using another repository containing the gripper. But not using the official ROS-i repository of Robotiq.

<?xml version="1.0"?>
<robot name="robot_system" xmlns:xacro="http://www.ros.org/wiki/xacro">
<xacro:macro name="robot_system_urdf" params="robot_prefix gripper_prefix robot_type"> 
<xacro:arg name="transmission_hw_interface" default="hardware_interface/EffortJointInterface"/>
 <!-- Robot -->
        <xacro:include filename="$(find ur_description)/urdf/ur10.urdf.xacro"/>
        <xacro:ur10_robot prefix="${robot_prefix}" joint_limited="true" transmission_hw_interface="$(arg transmission_hw_interface)"/>
    </xacro:if>

    <!-- Robotiq 85mm gripper -->
    <xacro:include filename="$(find robotiq_2f_85_gripper_visualization)/urdf/robotiq_arg2f_85_model_macro.xacro"/>
    <xacro:robotiq_arg2f_85 prefix="${gripper_prefix}"/>

    <!-- Joints -->
    <joint name="gripper_to_robot" type="fixed">
        <parent link="${robot_prefix}tool0"/>
        <child link="${gripper_prefix}robotiq_arg2f_base_link"/>
    </joint>
    <!--<joint name="${robot_prefix}world_joint" type="fixed">
        <parent link=""/>
        <child link="${robot_prefix}base_link"/>-->
        <!-- <origin xyz="" rpy=""/>-->
    <!--</joint>-->
</xacro:macro>
</robot>

The plugin for Gazebo is loaded in another URDF and both are merged in a final robot_system.xacro, which I am launching in Gazebo.

I am not able to load any phtos here but the result is similar to the one shown in here: link text

Does anyone know how to overcome this or can provide some information on how to solve it? Maybe someone has worked with both the Robotiq and UR repositories in Gazebo here?

I am running ROS Melodic and Gazebo 9.0.0

Thank you in advance!

EDIT: In case this doesn't work (I've been trying also with the arg2f_140 model), I am considering either using a non-official package (just like the one provided by WayPoint Robotics) or maybe the 3 finger gripper which seems to have the gazebo definitions requiered, just if anyone wonders out. Does anyone know about a working out-of-the-box gripper for UR for gazebo simulation and MoveIt configuration?

EDIT 2 (SOLVED): As I was not able to load the 2 finger gripper properly I decided to use the 3 finger hand, as long as I do not need a real robot to run my application, it is only for testing some software. At first I ... (more)