Depending on what you are planning to do, you either need to edit the GMF to include the objects at startup (then drop them using a python command if you want them to move or have mass so that they move right away) or add them to the arena from within the python (then wait for something to hit them if they are suppose to move). GIB and TXT files have nothing to do with objects.
In practice.py, you will find lines like these:
self.AddXtraSound("Obstacle", "arenas\\cones\\cones.gmf", "metal", "Sounds\\cone.wav") #(reference name, GMF object, sound for collisions)
self.AddXtra("Obstacle", "arenas\\ramps\\ramps.gmf", "metal") #(reference name, GMF object)