There are three sections to the test: Reasoning in Humanities & Social Sciences; Written Communication; and Reasoning in Biological & Physical Sciences. Most candidates are concerned with the third section because it assumes a certain level of knowledge (equivalent to first year university level chemistry and biology, and A-level/Leaving Certificate/Year 12 physics).
GAMSAT Preparation Sections 1 & II: the best way to prepare for Sections 1 & II is to read widely, read periodicals, read novels, read pretty much whatever you can get your hands on. Think about, and understand what you are reading. Look for the implied meaning in what you read. Form opinions, and judgements that you can then apply into interesting pieces of writing for Section II.
GAMSAT Preparation Section III: people prepare for Section 3 of GAMSAT in a number of different ways, for example: