OK well hopefully things will be a bit less confused by global pandemic by then anyway!
But I may have misunderstood - I was assuming she would be applying for places in the Lower Sixth to start a 2-year course of study. So (revised now I understand this is for September 2022 not 2021) aiming to take A-levels in the Summer of 2024 if she will be returning to England in 2022. Am I wrong? I doubt any school would take any pupil into the upper sixth with only 1 term of learning - the Spring term is revision, and the Summer term is exams, it would be impossible to catch up no matter what country you were transferring in from! If she would have been expecting to finish high school in 2023 not 2024 then no there isn't a way to "transfer credit" for work already done in order to not have to do the full A-level course. To get the qualification she will have to sit the exams and submit any coursework just like anyone else - but she could choose to study independently and take the A-level exams as an external candidate if she doesn't want to start a 2-year course of study. Alternatively, if it's not appropriate for her to enroll into the lower-sixth, it might be appropriate for her to skip A-levels and look towards getting a place on a Foundation Year at a university, which are designed for students coming to the course without having gone through the traditional route through A levels (usually for mature students).
But assuming that's not the case and she will be looking for a Lower-Sixth place, I don't see any major problems, and you have plenty of time for applications, so that will be good. You can and should make contact with all the potential sixth form providers in the area you will be moving to. Don't forget to check out colleges that specialise in post-16 education, she doesn't have to join a school that also caters for younger pupils. They will each have their own processes, sixth forms don't usually have a single application form administered by the LEA though some LEAs will be more help than others.
Whether there are gaps is going to depend on her subject choices. Subjects like Maths and sciences are going to build heavily on the foundations of what was covered in the GCSE syllabus and if the syllabus at her american high school misses something out, there will be a gap but nothing that can't be quickly caught up with if it's a subject she enjoys.
There are unlikely to be knowledge gaps with subjects like english, history, geography etc because the A-level syllabus will be covering completely different topics so as long as she has the basic skills from having studied them at high school she will be fine.
A lot of schools offer A-levels in subjects that are pretty rare to study at all at GCSE, like Economics and Psychology. If she is choosing subjects like that then everyone will be starting from nothing, so no gaps to worry about.