Sostenueto it's great the your DGD puts so much effort in, and her work ethic will serve her well, not just through the time when she is at school and university but for her whole life. There us much to be admired about it.
I am hesitating to write this next bit for fear of causing offence, but it is kindly meant.
The volume of work steadily increases from now onwards, along with the amount of material to be learned. From your descriptions, it sounds like your DGD pushes herself to learn every minute detail, which is laudable at GCSE but becomes unrealistic somewhere between Year 12 at school and the final year of university.
There were people at university with me who had always prepared for exams the way your DGD does, and who suffered emotional fragility when faced with the fact that there just aren't enough hours in the day to go over every detail again and again, the very strategy that had served them so well previously. Some would resist change and cling to their old habits, cutting out any social life, then even reducing the number of hours sleep they got each night. Then they get into a cycle of getting more and more stressed and falling further and further behind, until, finally they are actually ill. The adjustment to a strategy that works when the volume of work is high was really difficult for them and one girl whom I remember vividly just couldn't do it and ended up trying to overdose.
So, my (unasked for) advice would be to encourage your DGD to start learning now not fret if she can't recall absolutely everything and to teach herself how to differentiate what she is taught in lessons into vital, important, etc (every type all the way down to "trivial") and then divide her time accordingly.
I hope I have not offended you by writing that.