Consider that the weight given to GCSEs is primarily due to a lack of A level results (students only have predicted results when applying, which can be as optimistic as US GPAs), whereas OP already has their AP results in hand, which are the analogue to A levels.
When you mentioned the example of a 6 in GCSE maths, I disagreed as I believed a 6 in an unrelated GCSE would not be a devastating blow to an Oxbridge application. It took me a while but I did find some evidence (if you can call it that): Are my gcse results good enough for Oxford - The Student Room - and this is for a course that is more mathematically demanding than English.