As other people on here have said, a 2:1 is probably the minimum you would need. If you can get a 1st, then even better, but it all depends on the requirements of each individual University.
Main thing is to therefore do your research. Try to find out which GEM programmes need 1st class degrees, which ones require UKCAT, BMAT etc, and which Unis have very high cut off scores for things such as the UKCAT.
It sounds obvious, but to maximise your chances of getting into GEM, you must apply to your strengths.
For example, my UKCAT was very average so I avoided places such as Newcastle and Warwick, which have very high cut off levels for interview. However, my degree score was very good so that opened the doors to other Unis.