You don't necessarily need "excellent GCSEs". Several medical schools don't even score GCSEs (e.g. Imperial, UCL) and others may put less weight on them. You would need to at least meet any minimum GCSE requirements (commonly in English and Maths) so you may need to plan to resit maths. GCSE retakes are generally not an issue for medical schools.
You will need to do A-levels or an equivalent qualification (e.g. IB, an acceptable Access to Medicine course) though for applying to standard entry medicine. If you have left school now the potential challenge will be the cost of doing them, as medical schools require you also do the science endorsement (practicals) which is quite expensive to do as an external student (and can be hard to find a centre that offers them I understand).
Depending on how many years you are into your degree it may be that finishing the degree and applying for graduate entry medicine would take a similar length of time. Graduate entry medicine is more competitive however since some medical schools discourage applicants who are currently studying elsewhere (or won't consider an application at all, although this is much less common for people doing a degree that isn't a medical degree already), it may be equivocal.
That would also potentially give you longer to plan out what you need to do, undertake relevant work experience, prepare for admissions tests, and figure out if you do need to take any qualifications (e.g. A-levels) after the degree to apply (many GEM courses don't have any degree subject requirements, but of those that don't they often require either a) A-levels in the relevant subjects or equivalent or b) are GAMSAT unis which require you cover similar content to prepare for the GAMSAT).