Look, Judging by the fact my GCSE's are a week tomorrow, I take it that your first GCSE is French?
Although you are slightly behind, You need to understand that you know more than you think and It's just understanding your weaker areas.
Okay, here's a plan. Each week, I have have a total of 3 Exams, although in one week (Next week) I have 2 and the following week I have 4.
Split your Book into chunks and allocate days as to what Chapters you shall do.
E.g. In my French Revision book, I have 10 chapters and I shall be doing 2 a day, which leaves time to go over Vocab.
The day before the Exam, If you have study leave then Spend a total of 6 hours+ Just on that Topic to polish your weaknesses.
Always leave a week for each exam and on the day before the exam, dedicate all your day for the exam (take good breaks as you can lose your mentality).
It's NOT ABSOLOUTELY TOO LATE, but with a bit of dedication you can bring your grades up to an A.