I'm assuming your parents are determining the rules for this. So if you want this to change, you need to suck it up, buttercup, and have a reasonable discussion on missing a practice. If it was a competition, I could see how you would be upset.
However, your brother is competing in a game, so your parents are having you go to support him. I would imagine that he would come to a gymnastics meet / competition for you. My vote would be to go and support him.