American system to the English systemWatch
These circumstances are bureacratic nightmares, it might be easiest/simplest to just... take the GCSEs. Is it anything in particular that they want, eg proof of maths/English competency?
It's quite late this evening, but by tomorrow morning it's never a bad idea to phone some Universities you intend to go to and explain to the admissions tutor(s)/departments about your conundrum. I presume you're quite competent at subjects like maths/science, so you should find most of the content straightforward and won't lose a lot of time for learning/revising extra material.
English language may have controlled assessments and coursework, which can be a problem if you plan on taking GCSEs but leave it to late to let your exams officer be aware of what you want to do.