A lot depends on where you apply. Additionally with a course like medicine it is inevitable that many candidates will be applying. However, its your AS and A2s which count for the most meaning that GCSEs are really only going to come into play if there needs to be a shortlist due to too many potential candidates whom could quite possibly achieve high A level grades. I would go ahead and presume that if say Oxford or Cambridge were your desired uni then perhaps you should rethink your options as they usually require majority A* grades at GCSE, but, along with hard work and success at A level, you should be fine to go on to study in any top 20 uni should you wish.