Starchy foods like potatoes, pasta and sugary foods like candy, have a high GI they cause a rise in blood sugar. Proponents of these diets restrict foods with a high GI because they believe that too much blood sugar causes overproduction of insulin. Insulin, a hormone promotes fat storage. Thus, too much carbohydrate makes you fat. These diets place too much importance on the glycemic index.
Weight gain is more complicated than that.
A diet is misleading if it suggests there is one cause of weight gain for everyone. And remember, weight loss is still a daily balance between the total number of calories eaten vs. the calories used. Finally, carbohydrates have less than half the calories of fat. Whole grains like breads and cereals are high in fiber. Fiber helps lower blood cholesterol.