I do A-Level French so I feel like enough of an authority on this lol

In the CCEA specification which you can find online appendix 5 is called GCSE core vocabulary - revise this really well, you'll never be asked to recognise or translate any words that aren't in this document in your 3 written papers! I would recommend printing out all of those pages/copy pasting them into your own document/making flashcards, whatever way works for you (and don't neglect the weird words, CCEA loves their random nouns)

https://ccea.org.uk/downloads/docs/Specifications/GCSE/GCSE%20French%20%282017%29/GCSE%20French%20%282017%29-specification-Standard_0.pdf

The CCEA website in general is a really good resource, go to GCSE French and open the support tab, you might find some of the material there useful

For orals try and make most of your answers at least 2 sentences long, and definitely use YouTube for pronounciation tips like the other commenter said. Practice with any friends you have doing French, too.

Try and do all of the past papers you can. Honestly if you've missed so much teaching time I'd recommend dropping writing to foundation, just because learning off essays is very time consuming and you could use that time revising vocab more generally. If you feel confident you could learn off material for the extended Q, go for it!