PB Blaster or Kroil
I would either spray it with PB Blaster or pour Kroil down the tube myself. Pour or spray from either end as well and allow it to sit up for a couple of days. Hopefully by then you can drop it out by bouncing the gun or maybe use a thin piece of spring stock and tap it out.
Once you get it out, you are going to need to scub out the tube very well. I would suggest using either the Kroil or PB Blaster on steel wool and use a dowel to push the steel wool back and forth. If this still leaves some rust, I would wrap a sheet of fine emery cloth around a dowel and then use the dowel to push it back and forth, and then repeat the steel wool. Then I would use some 409 to wash it out and then blow it dry with an airhose. Make sure from there that you use a light touch of oil such as breakfree wiped on the new gas rod. I wouldn't even try to clean up the old rod if it is rusted enough to be stuck in the tube. You may end up with a little larger tube, but you should be ok with a new gas rod in the old tube.