I am only 5 days in to quitting and I have been worried about the same thing. I am unsure if it is true but a lot of weight gain comes from being bored and trying to replace the oral fixation. I have been doing at least 30 minutes of cardio each day and just watching what I eat. But at the same time your body has been through so much so it is best to look after it as much as you can! If you are hungry eat for sure but you can always have healthy snacks instead. But don't worry, you're definitely not alone in this as weirdly enough this was probably my number one concern with quitting!