Due to the fierce competition for places at university to study medicine, you will need a good range of broad GCSEs and having good grades in each of them.
More specifically the universities will look for a top or near top grade in mathematics. This is because it is a good indicator of analytical abilities which is essential to become a good doctor. Communication skills are a priority when medical schools are looking for suitable candidates and in this regard, you will need a top or near top grade in English language and English literature GCSE.
As well as Mathematics and English, the medical schools will be looking for high grades in the three sciences i.e. chemistry, biology and physics. If you feel that you have come up short in any of the above, you can still contact the university to see whether they would still consider you. These criteria apply to medical universities in England, Wales and Northern Ireland. Of course, students will have other subjects that they will have studied in GCSE such as history or French but the medical schools are interested specifically in the sciences, maths and English with the remaining subjects still having decent top grades.