Hmm - I sat the GAMSAT Australia in 2013, and I would have to say that most (but not all) of the Section 3 questions were indeed self-contained. It's been a while, but from what I recall, knowledge of the following would have been beneficial for some (but I reiterate, a minority) of questions:
Constant acceleration formulae from Year 12 Physics (xuvat)
One question required an understanding of forces applied on an axis
Markovnikov's rule to predict a major product
Some knowledge of DNA (base pairing and kinds of mutations)
The official ACER practice books also have questions that require more knowledge (e.g. of osmosis and diffusion). It goes without saying, that being passingly familiar with more topics is helpful. For example, in one of ACER's practice questions, they ask you to deduce the correct order of events that allows a person to breathe in. Now, at that time, I had not studied lung physiology (I would later that semester). However, it would still have been easier to answer the question if you are aware of the relationship between pressure and volume, and that pressure moves from high to low, where possible. Whether you know this from the general gas laws, some study of physics, atmospheric science etc., these kinds of knowledge make solving the question both easier and faster (because you don't need to 'derive' or use thought experiments to deduce the answer).