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I had a similar problem some time ago and end up with a recompose.py script. Please have a look at this question.

Basically the script allows you to convert any message to any just from a command line. For your case it will be something like:

$ rostopic echo /gazebo/model_states | ./recompose.py "{pose: 'pose[1]'}" | rostopic pub -r 100 /pelican/pose geometry_msgs/Pose

Although this is convenient for prototyping, for a production code it might be better to write a small node as @gary-servin suggests.