Your GCSEs have reduced your chances of attending Oxbridge, and of studying medicine (unless you take the long route). It is not impossible to do either, but your A levels and PS will have to be excellent.
Don't retake unless that C is in English Language or Mathematics, and it might prevent you from getting onto a course that has that grade as a prerequisite. Concentrate on your A-levels and don't get distracted, because you can have 10 A*s at GCSE but if you flop your A-levels and apply for Oxbridge with ABCD, it won't be worth it.