GCSE grades are important. They may say its only for college applications, but its the first thing that admission tutors will have to look at. They want students with consistancy in their work. In other words, they don't think highly of people who got poor GCSE results and good A level results. However grades only play a limited part of getting into medical school.
Don't believe your predicted grades, understand that these predicted grades are the lowest you can get. But most of the time, people get better than their predicted grades. For example, I was predicted to get 3 As, 6Bs and 3 Cs. Guess what I got in the end... 8A*s and 4As (not trying to boast, just trying to boost your confidence).
Applying for a medical school is very different to applying to a physics school for example. Because the people who want to do physics at good universities works very hard and reads a lot to gain their knowledge and the interviewers usually look for amount of knowledge, ability...etc. However for medical students, they assess you in 3 parts. Communication, Motivation and Well-roundness. Communication involves understanding the ethics and problems in medicine such as Euthanasia, Blood transfusions, HIV, current NHS problems, Private medicine. Even SARS will be a hot topic for interviewers this year. Motivation is where your grades and consistancy comes in, why you want to be a doctor, what are you particularly interested in medicine, and why you are interested in that part. Well-roundness, this is where they look at you overall including your extra-curricular activities, clubs, positions of responsibility e.g. working at care homes. Work experience, sports, music, arts, drama...etc. Each of those three parts are given a score out of 5.
So basically, having poor PREDICTED grades isn't the end of the world. If you are really serious about doing medicine as a potential career, then work hard during your A levels, push yourself to the extreme. In the end, your hard work will pay off.
Out of interest, why do you want to be a doctor?