I think it's a bit too early for you to think about it, but I'll be sitting it this coming Saturday and people have been asking me about my prep, I'll try give you an insight into how students study for it.
Students typically begin by revising their 1st year science knowledge (which shoudn't take more than two weeks) before they start tackling questions. It's also a good idea to begin reading a few books of literature and soaking in some new vocabulary when you start. Admittedly, I did this more at the beginning-middle of my prep but I've placed more time on doing more practise questions for the humanities section lately (haven't read a book since February, I just occasionally flick through The Economist and New Yorker now).
In my opinion, Humanities can be improved simply by hammering lots of practise questions and analysing why your choice was wrong. I think this helped me more so than just reading endless amounts of literature. Its sort of like advanced reading comprehension in a way. Lots of people get questions from Des and the ACER tests - if you do them over the summer holidays then that'd be great. Examkrackers MCAT has a different style from GAMSAT in my opinion, but it's a good book to start off with. I did maybe eleven out of the fourteen tests. I started off at around 45/75 when I first tried a sample S1 test in December, but it's slowly improved to ~ 57/75 in the most recent ACER tests. There is also data-interpretation here, so get used to reading tables and graphs.
The essay section is probably the most loathsome to prepare for. I feel like shit immediately after every practise essay I write. There's so many prompts that the examiners can hit you with, so you're writing with a new idea almost each time. Write as many timed essays as you can, giving yourself 25-30 mins. Practise on both planning and writing. After you've written your essay, get others to critique it and trash it. Try to critique it yourself as well, making annotations and notes of what you think works/doesn't work, and if you want to, edit it as well.
I remember starting off writing 2 essays a week in early January, then up to 3 by mid-late January, then 4 by early-mid February, and now around 5-8. It's not much time though, really. Investing two hours a week already earns you four essays. I guess reading and critiquing other people's essays also gives you a more critical eye.
For the argumentative essay, it's good to have a good background knowledge of current affairs to draw your examples from. Tbh I watch ABC News 24 and randomly surf through Wikipedia all the time, so that really helped. But I think some students make a mistake when they focus on just solely arguing with examples throughout the entire course of the essay. A GAMSAT assessor told me that what they look for is logic. They want to see that you can build a coherent, logical argument, using one piece of logic to step onto another. After that, you can illustrate it with a fleshed out example. Try aim for cohesiveness and flow throughout your essay as well - make sure everything links together. Some students will tell you to write with a "for-against" sort of structure, but there's no set structure for you to adopt.
The discursive essay places more emphasis on personal reflection. This is the essay where you should draw your upon your own feelings and just write honestly, "from the soul". The above assessor told me that examiners love feeling a connection to the writer - its tough to do, but if you pull it off, you can really lift up your marks.
These essays are inherently different from the ones you write in high-school and in university. You don't have enough time to back up your thesis with a whole bunch of supporting evidence, you need to focus on ideas and thought. Picking a writing subject at university may help with expression but I don't think it confers a significant advantage.
Finally, the science section is probably the easiest to prepare for assuming you have science background. A knowledge of 1st year Biology, Chemistry and Year 12 physics is necessary for you to understand the questions.
In all honesty, revising Biology is a waste of time. Yes, this section is knowledge-based than the other sections, but ACER will hit you with content that you've never studied before. i.e the physiology of a bird's respiratory system. Or wolf packs and crap. You just need to know the basics of Biology imo (Year 12 stuff in my opinion - osmosis, biomacrmolecules, organelles - just be aware of them). Most Biology questions will involve the use of graphs and other visual information.
Having a good knowledge of Chemistry is very advantageous. Invest most of your prep on 1st year organic/inorganic questions. The ACER tests will sometimes give you 1st year questions that you've studied before (stereoisomers and the sort are free marks), but most of the time they will give you an example of a mechanism/reaction and ask you questions based on it. Do not rote learn your 1st year chemistry - understand the concepts. Most questions will be unfamiliar to you, so having a good understanding is critical.
Physics isn't tooooooo bad, you can probably get by with Year 12 knowledge but perhaps 1st year Physics will help you to an extent. What is considered to be "Year 12 physics" differs between states, so don't assume it just means VCE physics. This section involves the most maths (without a calculator - so get used to estimating and dividing decimal places. I find that practising scientific notation helps here - dividing 0.48 by 0.000000096 is much easier as 4.8E-1 / 9.6E-8, so it's really a matter of index laws).
As said, many of the science questions will be unfamiliar to you. This is why you shouldn't spend too much time revising up in this section. There is more emphasis on "Reasoning" than "Knowledge". ACER will give you enough information to answer the question. Many students may find themselves getting 50-70/110 on this section, but if you want to get in the top range then you must aim for 75-80+/110.
Ok, now many students ask about prep courses but I don't think you need to attend one. The only material I bought was Examkrackers, Des' O Neill MCQ questions and the four ACER tests - I didn't attend any course. I also paid above assessor for his critique on my essays. Many people will sign up with companies such as Prepgenies, Des' O Neill and GradReady - they have lots of practise questions and will mark your essays.
And yes, GAMSAT is nothing like UMAT. It tests very different things. I've seen people who got 60th percentile in the UMAT get above 90th percentile on the GAMSAT.