Different medical schools place different weight on the significance of GCSE results.
Oxford, for example, place great significance on them and choose applicants based on the proportion of A*s achieved at GCSE (in conjunction with your BMAT score and personal statement). I believe Kings and Queen Mary's care a lot more about your UKCAT so you can compensate for a weak GCSE performance by having a high score in the UKCAT.
The key to getting a place is to apply to your strengths. If you don't get amazing GCSE results but end up getting a great UKCAT score apply to a uni that places more emphasis on the UKCAT. One of my friends, who went to QE grammar school but only achieved 3 A*s,did this and he got a place at Queen Mary's. I on the other hand achieved 8A*s at a pretty average state school but flopped my UKCAT so I applied to uni's that favoured GCSE's/ required the BMAT and I got a place at UCL.