music - gsce and A levelWatch
It will be your A level subjects and grades that determine your university entrance. Saying that I would expect most A level Music candidates to have done gcse.
If you did end up not doing either, it wouldn't be the end of the world. Most unis don't ask for it (though some do, I know Surrey does). If you want to do music at uni though, I would really recommend it.
Some other stuff unis ask for is a certain level of practical and theoretical ability. Normally grade 5 theory (possible higher if you don't also have the a-level) and grade 7-8 practical (normally grade 8 for Russell Groups). Lots of unis also ask for an audition/interview.
When you say music uni, do you mean conservatoire? They have fairly different course content suited to different types of musician.