Controll of urdf on gazebo with ros topics

edit

I've managed to create an urdf file and can simulate it in gazebo. It consists of a 3 finger gripper so the joints are all revolute. Now I want to be able to rotate them on gazebo or rviz, this is, open and close the gripper, with a ROS topic. How am I able to do that? I cant publish the topic but can't make the model move since I can't make it subscribe the message. I think that I have to use the topic "hand_command" and the msg "trajectory_msgs/JointTrajectory". Is there any tutorial on this or can anyone give me some help? The gripper is based on the model found on the link of shadow hand smart grasping sandbox.