What is a credit equivalent to at GCSE?

There isn't a direct equivalent of a credit at GCSE level in the English system. GCSEs are graded, typically from 1 to 9 (with 9 being the highest), or sometimes A* to G (with A* being the highest, though this older grading system is being phased out). Credits are a grading system used in some other countries, like Canada, and are not directly comparable to the GCSE grading scale.