There's too much of an emphasis on diets; what to eat what not to eat, yeah there are lots of sugary or high fat foods that are not ideal, which only adds to our fat stores when over indulged.
The key to maintaining an ideal or desired weight is to match food intake with a level of activity, thus preventing the weight gain in the first place, therefore don't over indulge or drive everywhere and normal everyday activites will be enough to maintain a correct height to weigh ratio, without the need to participate in additional exercise or attending a gym.
If not due to an underlying medical condition; any weight gain, well the acquiring of excess body fat or belly flab,, is due to eating more than the body can burn off.
I eat everything diet books and nutritionist advise against but I don't over indulge, I only eat the correct amount for my size and current level of activity,
In regard to diet books, latest fad diets, celebrity nutritionist & their endorsements to sell DVDs & books, as well personal trainers, qualified or not, it's not always wise to believe the hype.