It's an interesting question. I read something on another forum once that summed it up pretty well; when you sit the GAMSAT, it will be the hardest exam you've ever done but looking back you'll realise it wasn't that bad.
Comparing it to the UMAT...I spent Jan/Feb preparing for the GAMSAT, so it's definitely something that can be studied for. Whilst the science section does place an emphasis on questions that require reasoning rather than memory, having studied the material is definitely a major bonus. The first section is the closest to anything you'll see on the UMAT, the majority of questions will have to do with reading passages and answering questions about them. Most of the passages are a pretty high level of english, but it's not impossible to study for it. The second section is the writing tasks and, just like VCE english, you can write essays to study for it. The topics will be similar to the stuff on the GAT.
Since section 3 is double weighted, you can do well enough there to compensate for poor scores in the other two sections...that said, they will look at individual section scores and UoM refuses to double weight the science section (which I actually agree with). Someone in my year got a 50/54/100 for his section scores, which gave him an overall score high enough to get into MD (pending interview). Background knowledge in the sciences is necessary to be able to do the questions on section 3. You WILL need to have done chemistry in uni, you could probably scrape by with biology and physics at high school level.
If you put in the effort for the GAMSAT you will see results. Some people do ridiculous things like studying for several months full time, but you don't need to go that far. I wrote a dozen sample essays or so and revised all of 1st year chem/bio/physics (which sounds like a lot, but over two months it was only a bit a day). I have a ton of study stuff, PM me if you want to see a sample paper or response or something, I've got a lot of it floating around my external HD.
physics in it can be covered in uni?
Yes, of course it can. I never did VCE physics and did well on the gamsat. There might be a few bits and pieces that slip through, but you can either study them individually or not worry (there probably won't be many questions on them)