Not sure if I fully understand your case. My first question is: do you need ROS at all for your use case or you just want the Gazebo simulation and send the data to other application?

If you don't need ROS one option could be writing a standalone application that subscribe to /gazebo/default/joint gztopic and in the callback transform the data from gazebo.msgs to a given format useful to send you to your application in python (not sure about the best way here but there should be plenty of options). To implement the application you can look into the stand_alone example of listener and change the code in the callback.

Another option using ROS could be to integrate rospy code inside your python code to subscribe to the joint_state ROS topic and use the information directly from there. This is probably the easiest approach.