I haven’t read all the responses.
You do not need to explain your GPA. Doing so will sound like an excuse and/or will not endear you to the admissions officers by drawing attention to something that you perceive of as negative.
Your piano skills will obviously be a large part of your app and they have seen performance level musicians before. They will be aware that you spend many hours practicing, as do other types of musicians, athletes, and others who have extremely demanding extracurricular activities.
You have what you have. It’s up to the college to decide if they want you. You can describe your piano commitment in more detail in the additional information section. No doubt you’re probably submitting clips of your performances too.
You’re right, it sounds arrogant. Don’t confirm it by trying to explain what needs no explanation. You still need great recommendations and course rigor if you’re aiming high. Good luck.