Nah, apprently it isn't. Your expected to have some knowlage of Science (which the GAMSAT tests) but other than that, nothing is assumed. And the GAMSAT is suppose to be Year 12 level science.
But I've been told (by Sydney Uni Grad. Med department) to do a degree that intrests you, and you will perform well in. They have drawn people from Arts, Law, Science, Allied Health, Nursing, Education --> every faculty. Nobody has a clear advantage over another based on their undergrad degree --> everyone has strenghts, but thats it.
Hope that made sense..