I'd contact uni's asap and ask about dropping out. You've yet to complete a single semester. Some certainly do accept students who left another course and a very select few accept students who are currently on a course. The only caveat to this is if you were already on a medical course, which you're not so it's a moot point.
I'd advocate this because you have the grades to get yourself to interview, the only other aspects are work experience, volunteering and entrance exams which can definitely be built on significantly in a years time. Also, the competition is far lower for the A100 courses than the grad courses; there are twice as many A100 courses as GEM and cohort sizes are much larger. Not to mention the chance that the grad entry course may be scrapped.
When you get to GEM your A levels matter little and your GCSEs even less so. You'd be at no advantage going the GEM route as far as I can see, excepting a potentially wider/different choice of medical schools, but if you do go that route then aim for a 2i classification minimum in your BSc.
I'd be more concerned for SF graduate entry wise than dropping out and getting on an A100 course. Finance is a nightmare for most GEM students. There is funding currently but not even 2016 entry students know if they'll be funded or not.
Not to mention if you do get on the A100 as a graduate you only get maintenance loans, so even if you lived frugally you would need to find £36,000 in fees.
In my opinion there are too many unknowns in GEM to pin your hopes on it now when you have a good set of grades to apply for the A100, just find out who will accept an application from you. Not all uni's see it as being inconsistent and indecisive. Indeed to drop out of uni because you've decided it's not the course you thought it was would show some courage, no? Just depends on who views it that way.
Also ignore the moron saying grad entry is less competitive than A100. School leavers with grades are not given priority, that defeats the purpose of a unified and fair application system. What utter *******s.