Is it very unlikely for someone with ABCab A level grades to be accepted onto an Oxbridge Medicine course as a graduate wishing to do GEM?
It is unlikely to get to Cambridge GEP without outstanding A-levels. There was a freedom of information request somewhere showing that the vast majority of accepted students had AAB. It's not impossible, however, and an outstanding university result and extracurriculars (academic conferences, publications and so on) will help you.
Oxford are a bit more lenient and your A-levels are less important. They'll put more emphasis on the BMAT however.