GCSEs are a memorisation exam. If you are able to learn and recall everything in those CGP books, your exams will be a piece of cake 👍
There isn't anything in the exam beyond what's in the CGP textbook. (BTW I am only mentioning CGP because I'm not sure if this is the case for other textbooks, and CGP does an amazing job of cutting out all of the unnecessary nonsense and only explaining what you need to know for the exam).
No point doing exam practice until you know the content! Patch up your knowledge gaps and then begin the exam paper practice grind