You can do medicine with combined science GCSE. As for GCSE grades it depends on which medical school you want to go to, for example, you would need all 9/8's to apply to Cardiff while qmul only requires 777666. Medical schools also look at your UCAT/BMAT scores as well as personal statement and work experience. Medicine isn't an A-level. You need biology and chemistry for most medical schools and your third subject can be any subject you want unless you want to apply to Cambridge for which you will need maths or physics.