Have you spoken to your coach? As a gymnastics coach myself I would wonder if your coach has been doing an muscle building conditioning. Increasing muscle mass could explain some extra weight
You should ask your parents to take you to the doctor and have your thyroid tested to make sure it's not under active. Under active thyroid causes metabolism to slow down, and is a very common cause of sudden weight gain. You need to ask for the following tests: TSH, Free T3 and Free T4, along with thyroid antibody tests, Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TgAb) to determine if you have Hashimoto's.
You could lose 2 kg in a week, but you'd have to be very diligent and it would only be possible if your whole body is working as it should. You can try varying the number of calories you eat daily, but since you are very young and not fully developed, don't decrease them too much. You should also vary your exercise. Once your body becomes used to certain exercises, it comes to expect that amount of exercise and it's no longer a challenge. If you're doing your gymnastics, daily, try adding other exercise with it, that you aren't used to doing.
Enlist the help of your parents to make sure you're eating healthy meals and getting enough rest.