A GCSE is a General Certificate of Secondary Education. Unlike your scholastic system, you get individual GCSE's for each subject.
So you get one for maths, another for English and so on.
The problem with a single certificate you get is that NATS require specific grades for English and Maths, where as all you provide is a general certificate covering all subjects.
If you do need to take the GCSE you will only take one, you don't need to do all subjects as in France