I've actually never encountered a graphics A-Level but I can still give you some (semi) helpful advice. Whether or not you can study certain subjects at A-Level if they haven't been studied at GCSE is entirely up to the school you attend. For certain subjects (like the sciences, MF languages, maths/further maths etc.) you'll typically require previous attainment at GCSE to a certain standard (or, in the case of languages, demonstrate some proficiency in it if it's, say, your home language). This standard varies - usually the best sixth forms/colleges etc. require 7-9s/A-A*s but the "not so good" ones may ask for something significantly lower, like a 4/C. However, with some A-Levels that are uncommon/not offered at GCSE, they usually won't require previous attainment. Graphics is a funny one - I'd imagine that they wouldn't need a Graphics GCSE since they're quite uncommon (at least, to my knowledge they are), but they might request a specific grade in a related subject, such as art, or a subject that has useful, transferable skills, such as maths.
In short, look at the entry requirements of your specific school and see what they say. If they do require graphics, some schools may be understanding to some circumstances, eg. your secondary school didn't offer graphics/you chose it but there weren't enough places etc, so don't be afraid to get in touch if you have to.