The GMC website contains lots of information about the Council's procedures, and how to raise concerns with them. You can also contact the GMC by post:
General Medical Council
178 Great Portland Street
If you have a complaint about a doctor, the fitness to practise department can be contacted by telephone on 020 7915 3603 or by email at firstname.lastname@example.org.
If you have a more general enquiry, you should call 020 7580 7642, or email email@example.com.