I believe most GCSE rubrics state you need to write in black ball point pen. Gel pens and other colours and media are usually not suitable for various reasons (scanning poorly, prone to smudging, being erasable in the case of pencil, etc etc).
The may note that certain other media are suitable for specific questions on specific exam papers potentially (e.g. you may be advised to use colour or pencil for drawing diagrams or graphs in some exams, although I think a lot now advise to do it in pen).