XACRO： Acutator use in gazebo turn robot to a strange performance

I am not sure whether I can state clearly. I will try my best. I used ROS & Gazebo to do my Robot Simulation I follow the Tutorial of URDF to the gazebo part. However, I found if I add the following part transmission_interface/SimpleTransmission100PositionJointInterface Then I found the robot in the gazebo seems won't be affected by gravity (seems like turn g from 9.8 to 0.5) .. very strange.. However, if I remove part: 100 Then the robot in the gazebo will be like in the real environment (which means g is 9.8) I try to search via google but failed. Does any one know why? Thanks very much!