This question has been asked millions of times and my short and simple answer is...do anything (preferably science related as this gives a slight advantage in the GAMSAT). You see, there's no advantage in doing BMedSc over BSc (Adv) or whatnot. Just keep in mind that you can think of BMedSc as a BSc with compulsory units and therefore it's not too great unless you're satisfied with a research career should you fail to enter via GAMSAT. As for Advanced Science, it takes longer and is harder, but you get scaled more. All in all, take the one which appeals to you the most in terms of structure andcontent and length of duration. Another important question to ask is whether you'll be satisfied with the degree if you don't end up in medicine.
You should also remember that the GAMSAT is similar to the UMAT in that logic and deductive reasoning are essential. You must read long tracts of imformation and respond to short answer questions in a limited amount of time. In that sense, I personally believe that a humanities component would be highly advantageous as it equips you with such skills and indicates to the university that you have a broad perspective and interest in the world. Courses such as BSc/BArts of BLiberalStudies are good for this.
