How to set mimic joints for gripper simulation in Gazebo?

I know there are some older answers and Github issues about using mimic joints to simulate grippers in Gazebo, but most of them appear to be outdated. I am trying to create a Gazebo simulation for a Robotiq 2f-140 gripper with ROS kinetic, but the joints do not move as expected, and result in a gripper that looks broken when trying to move it.  I am using the [urdf files](https://github.com/ros-industrial/robotiq/blob/kinetic-devel/robotiq_2f_140_gripper_visualization/urdf/robotiq_arg2f_140_model_macro.xacro) from the ROS-industrial robotiq package which specifies mimic joints, and have added the mimic joints to the transmission file `robotiq_arg2f_transmission.xacro`: transmission_interface/SimpleTransmissionPositionJointInterface1${prefix}finger_joint${prefix}right_outer_knuckle_joint${prefix}finger_joint${prefix}left_inner_knuckle_joint${prefix}finger_joint${prefix}right_inner_knuckle_joint${prefix}finger_joint${prefix}left_inner_finger_joint${prefix}finger_joint${prefix}right_inner_finger_joint The gripper still fails to move as expected though. How do I use the gripper in simulation?