Hello,
Whatever the diet is, once you are off it, it is up to you to keep up the good work or to quit on yourself. That being said, if you go back to your old bad eating habit without taking in consideration all what you learnt from doing your IP protocol and going through ALL the phases which is a very essentiel key to success, then yes you would gain back, but then again... would it be the diet that failed? No... It is up to us to do all the phases as we are supposed to, and once ''over'', nothing is over, it is a lifetime challenge that we would have the tool to go through as smoothly as possible.
What I am trying to say is that if you do all your phases as you are supposed to be and once you're ''off'' the diet you pay attention to what you eat, no, you would not gain the weight back.
Good luck in your weight loss journey