Is cold water or cold food causes weight gain?

Not usually: The temperature of the food does not cause weight gain. Weight gain depends on how many calories are in the food, and how much is eaten. If food is very hot or very cold, a person can't eat hardly any of it... Which means few calories get eaten.

No: Weight gain is more calories in than out, rather than the temperature. Maintaining a healthy weight is difficult enough without these "pop" superstitions. The best thing to do is get aerobically fit and avoid greasy / salty / sugary foods that folks tend to overeat -- soon these will become unpleasant for you. Good luck.