Gaining

Why not both?

as long as you go heavy on the carbs and calories you should gain quite well; whether it is sweets or other stuff; if you are going heavy on the sweets be careful as bacteria in the mouth produce acid when exposed to sugar and can cause tooth decay; so be extra diligent with brushing teeth and dental visits and stuff. Happy gaining!

Because of a bellyache after?)

Gram of fat has over double the amount of calories than sugar/carb (9cal vs 4cal). And diving into the sugar will, no joke, lead to diabetes pretty fast. If you're going to splurge hard, stick more with your fats: butter, cheeses, cream, oils, peanut butter, nuts, etc.