There is not a huge jump in my opinion in terms of the difficulty in the level of the language. You of course go deeper into some grammar points that your teacher might not have done at GCSE/only touched on at the end, like the passive and subjunctive. There obviously is new vocabulary to learn but, unlike some subjects, all your knowledge from GCSE is useful because you will need to know all the vocabulary you learnt at GCSE plus new stuff. The biggest difference for me was the content- at GCSE you learnt grammar, vocab, and key skills, and perhaps touched on some French culture. At A-Level, the grammar, vocab, and key skills are not what predominantly make up the content, they are incorporated into the lessons which focus on topics. These topics, unlike at GCSE, are not just about forming an opinion (e.g., what do you think of your school day/what do you do to protect the environment) they are learning facts about the culture of France and French speaking countries. I do Edexcel, and cover 4 themes, with 3 units within each theme.
Theme 1 incorporates sociology in a way, as you learn about the changes in family structure and marriage (unit 1), education (unit 2) and the workplace (unit 3).
Theme 2 incorporates elements of music and politics as well as sociology looking at French music (unit 4), media (e.g., whether there is freedom of speech in France etc.) and festivals and traditions (unit 6).
Theme 3 incorporates a lot of human geography and politics into it, looking at the impact of immigration (unit 7 and 8) and looking at the extreme far right in France (unit 9).
Theme 4 incorporates a lot of history with units 10-12 looking at France during WW2.
One way to look at it is that during your speaking exam at A-Level, your opinions have to be formed more on fact than just justifications that you can make up, because now you are being asked questions on things like.
And just when you thought you couldn’t incorporate more subjects into one A-Level, you also read a book in French which is essentially like doing English literature (we did Un Sac de Billes) which for me was quite a big step up in terms of your level of reading, and you also study a film (we are going to do La Haine) which is essentially like doing media studies, and you write an essay on a book and a film- at least with the Edexcel board. These essays are a completely new element for the A-Level which you will not have done at GCSE, and I would say that is the biggest step up. Of course, at GCSE you will have done a writing exam, but the essay is not just writing about how your nan broke her leg whilst on holiday and you missed your flight, now it is about how the writer uses motifs to represent themes etc. etc.
TLDR; content isn’t harder and the step up with the language is really manageable, but French (and any language A-Level) is more content heavy than one might think because it is incorporating vocab and grammar along with actually learning a lot of new information about many things to do with French society and culture, and learning a book and a film as well. I would 100% recommend it though, and it will look really good to employers because not many people do language A-Levels anymore.
Hope this helped.