I would talk with your primary care doctor
about these issues. I say this because it is very unlikely that an active gymnast of your age would need to lose weight or go on any sort of diet. In fact, I assume that you are actually consuming considerably more calories than the average person or even the casual athlete, and so you may need to increase the amount of food you are eating rather than the other way around.
I am concerned about talk of dieting in someone who almost certainly does not need to do so. Your doctor will be able to assess your overall medical health and determine whether you are at a healthy weight. Being underweight can predispose you to many long term complications, including osteoporosis and other conditions.
If you find yourself thinking a lot about your weight or restricting your food consumption to stay thin, or if you ever take laxatives or diuretics to lose weight, or if you ever have vomited to control your weight, then these would be very dangerous signs of a potential eating disorder
, which could be a serious medical threat to your health, and I would recommend talking with your doctor immediately.