I haven't done it but it is much more of a bitch than the umat.
Not to denigrate the UMAT but this is an understatement. Arrive at 8am, leave at 6pm. And I sat it on my birthday!
Otherwise what kingpomba said is pretty solid. The physics stuff was odd, some was very basic and some I'd never studied, so make sure to check the sample papers and revise all the concepts presented. If you only do uni physics, be careful, they don't teach everything at first year level so you might need to go back and revise it.
To do well requires effort and study. The UMAT tests more abstract reasoning skills, so there's (theoretically) no need to study in order to achieve past the cutoff. The GAMSAT, however, has a double-weighted section 3 that is all about what you know. It's true that most of the questions can be solved with a bit of logical thinking, but you still need the knowledge to eliminate the incorrect options. I studied over the summer holidays and did well, some people say that you need to start working well before that but I'm skeptical.
In terms of Biomed and Science...biomed covers all the (scientific) stuff on the GAMSAT pretty well in first year. Science has the potential to, but only if you choose the right subjects - chem + bio + physics 101. If you pick them, then there wouldn't really be a difference between your background.
The single biggest tip I would give to students sitting the GAMSAT is:
start reading literature now. If you want I can post some of the stuff from sections I and II - having a strong English and Literature background will REALLY give you an advantage. Even if it's just half an hour a day, read it now, it will help so much in establishing meaning.
Perhaps you may want this and theres anecdotal evidence from someone from this forum that they think biomed melbourne students tend to do better than science students
Wouldn't be surprised, but I think this is down to the fact that the average biomed student is smarter than the average science student (compare the course requirements), rather than how good the course is.
Science however you can however choose pretty much everything you want and all the subjects biomed students can ( i think.. dont quote me on this)
Science kids can't do any of our subjects, but we're allowed to take subjects from science (if we have room for them).