basically your undergrad degree has to meet the GPA cut off (distinction avg) and the rest relies strongly on your GAMSAT marks and interview.
Its better i think to do a health science, so that you are prepared for all areas of the GAMSAT... I B Science will make you go well in the basic science area but not so well in the humanities/communication parts.
You only need yr12 physics, 1st yr bio and chem for great scores in the science section of GAMSAT, and a B health science meets these requirments no worries. As well as giving you the clinical and human interaction experience you wont get in B science
thats just my personal deliberation, and why i am doing an undergrad health science.
the end.