I'd buy the sample exam papers from Acer so you're familiar with the format of the test, but you really shouldn't bother with anything else if you're from a science background. The science isn't 1st year uni level (in fact, it's barely even A-level standard).
The only reason you would have to revise for the science section is if you didn't do Physics A-level, but that only accounts for 20% of the science section. For the 80% that's Chemistry/Biology, all the information you need is in the question. The Physics questions do need some outside knowledge (I had to brush up on some classical mechanics formulae), but it's still pretty straightforward.
In the GAMSAT, they're really not looking for esoteric knowledge. They're measuring your ability to analyse and deduce information (Sections I and III), so there's really not much you can revise (I did 3 days of preparation and got 81 in the science section). For Section II, it's just a case of reading and writing a lot, forming opinions and structuring them in a coherent way. If you can do that then you can do well.
EDIT: Just so I don't give people the wrong impression - this only really applies to people from a science background. I can't really give any advice to non-science students, but I'm certain humanities folks will have some kind of an advantage in sections I and II. Section I is just textual analysis/verbal reasoning, section II is two essays (they ask you to discuss a number of quotations on a given subject).