In answer to the original question, yes you can. As long as you have or will be completing a bachelor degree in the year that you sit the GAMSAT, you can apply using the GPA from that degree.
I wouldn't necessarily choose a degree in the hope that it could potentially maximize your GAMSAT result. Firstly, its a three year commitment. You may change your mind about medicine after doing a year of philosophy or Japanese. Secondly, the science section is only ONE section in a THREE section exam. Reading/comprehension and essay writing are equally important which you may not get as much practice of if you complete a pure sciences degree. Finally, you should do a degree in an area which interests you - that way, you can try to maximize your GPA.