Hola! I did French and German post-GCSE at A Level and would say that I didn't find the jump from GCSE to AS too huge as a lot of the topics were similar to those in GCSE, but more in-depth and more independent study. AS topics will include technology, youth culture, holidays, family, friends + relationships, health etc - topics that we know quite a lot about which makes them not only easier but more interesting!
I also started Spanish AS (+ then continued to A2 this past year) from scratch which was a real challenge to begin with and I was at a disadvantage with a lack of grammar, but after the first term of AS I felt at a similar level to the rest of the class.
At AS you build on your knowledge at GCSE and develop your understanding of grammar, but a lot of schools (including mine) actually spend the first term recapping GCSE stuff to get people up to the same level, so I wouldn't worry too much!
To be honest, I found AS to A2 more of a jump as you deal with more ethical/political/social/cultural issues, but A2 is definitely the most interesting and it prepares you for degree-level language study.
I was also worried about having to write timed essays at A Level as opposed to spending ages getting them perfect and then memorising like at GCSE, but the more you practise, the easier it gets. Essays at A Level are actually quite short & a similar length to GCSE I believe - 250-300 words usually. You should get a lot of practice + help with essay structure which definitely makes the exams less daunting! At my school we wrote one essay every other week so that meant that we were much more used to thinking quickly for the exams. But it's not too scary because most exam boards give a choice in questions (eg between film/music/holidays essays) so there's bound to be something you like/can do! I know I'm biased, but one thing I would say about having a language to AS level is that it will undoubtedly be useful whatever you decide to do, and will attract employers. Plus, it could open up opportunities for studying/working abroad. Plus it's FUN!
Sorry for the essay - if you have any more specific questions about doing a language/Spanish @ A Level, feel free to ask me