You really shouldn't be stressing about GCSE's in year 8 don't worry! Just focus on being on top of your subjects now and maybe it might help (and would look impressive on your application) if you maybe thought about doing some volunteering? Maybe just in a charity shop or in a local primary school. But that really isn't necessary until like yr 10 or 11 - it might just be something fun to do in your spare time which builds on the qualities a medic needs.
Also does your school offer D of E? That could also be something fun to do and you get a qualification out of it? Also don't drop any sports, musical instruments/ hobbies you have they're a good way to relax and can be something to talk about in interviews.
This is such a long way away though, most medics don't even know they want to do medicine in yr8, just carry on being as determined and hard working as you are now and you'll be absolutely fine!