The gas comes from some of the electrolyte converting from a liquid to a gas. At that point, there are areas in inside the cell that can no longer conduct electron flow from the anode to the cathode. That means all of the current has less area in which to travel back and forth in it, and the current will cause that cell to run warmer than other cells from the same batch and type.
A pouch cell running with hot spots inside it will puff again. I suppose you could use it to make a low-amp device, like a 12V brick that extends the run-time of your laptop or cell phone. I do not recommend pulling high amps from a puffed cell that has been repaired.