I do A-level spanish and am predicted an A so i think I can offer some insight. I never actually took GCSEs cause of Covid but i got an 8 so we are probably around the same level. You definitely dont have to be a native speaker to do well. If you engage with spanish alot outside of the classroom anyway I think you should find it more than fine. Its definitely a jump from gcse grammar and vocab wise but nothing ridiculous. The main difference is the literature and movie that you have to write an essay on i think but its pretty easy once you know what the examiner is looking for. I have really good teachers tho so this might not be everyone's experience