Unfortunatley I don't think there is very much you can do to prepare for GAMSAT.
You should still see the practice questions so you know what to expect, but unless you are hopeless with basic sciences, there isn't much you can do.
The first two sections of the GAMSAT test your language skills.
The first part will be comprehension, where they give you a poem, a news article or an excerpt from a book. They will give you MCQs on "what is the intention of the author in the lines 15-18?". Basically like highschool English exams.
The second part is where you must write two short essays in one hour. Each one should be about 1.5-2 pages. They will set a theme and give you (for example) quotes from various famous people. You can choose one or all of the quotes and do an expository writing piece. The year I sat it, the questions for the two essays were social welfare, and violence in sport.
Understandably, it is difficult to improve your comprehension and writing skills in a short period of time, unless you do not even know the basic format of an essay (e.g. intro, body with arguments and supporting evidence etc, and conclusion). But I doubt this applies to anyone here.
The third part (science MCQs) is where you can do some limited preparation, for example if you are a history major and haven't taken science subjects since highschool. You are expected to be 'familiar' with 1st year level chem/bio and 12th grade physics. In most questions, you are given a bunch of information, and you can work out the answer from it. Of course it helps if you are familiar with the background info already. It is the logical reasoning process they are testing for, and not whether or not you know a scientific fact.
GAMSAT is not a terribly difficult exam, particularly because they do not set high cut-offs. It is merely a tool to screen out those who are bad, not to select those that are exceptionally good.
GAMSAT was brought in due to the fact that in the old days when the UAI (basically like your SAT scores) was the only thing that mattered, there were a lot of students getting in with quite poor English despite having near-perfect scores in maths, physics, chem etc.