(Original post by Sinatrafan)
I've sat the GAMSAT twice so I have a fair idea of what's entailed.
You sound like you're pretty set, what with having a biomed background. The science sections are by far the hardest and most important (they're double weighted on the scoring), so you are in a far better position than most of the non science grads.
I wouldn't worry about the physics too much, very little actually comes up in the exam and it is of a lower difficulty than the biology and chemistry. I think a good A-Level physics book should see you through that.
If you already have a background in chemistry and biology then the hardest thing will be first of all refreshing yourself, and then practicing as many questions as you can. The GAMSAT is very problem solving based rather than repetition of knowledge, so getting use to the questions is by far the most important factor. A lot of information is given in the questions themselves, which can help point you in the right direction.
I hear the Griffiths GAMSAT book is very popular, and of course the official practice papers you can buy are extremely useful. Aside from that, any good degree level text book should serve you well.
I'd say 3 months is a good prep time, and unless you are totally hopeless at reading and writing then section 1 and 2 need little preparation, aside from the reading a newspaper the night before.
I have to confess though that I found Section 1 and 2 very easy coming from a social science background, and that was the consensus among those I spoke with from that sort of background. On the contrary those from science backgrounds found section 1 and 2 hard, and the science straightforward. So take my advice about how little prep is needed for the first sections with a pinch of salt!