They come in two volumes (AS in a week and A2 in a week) and are summaries of the key facts:
Amazon.co.uk: AS In a Week: Biology (AS in a Week): Books
Amazon.co.uk: A2 in a Week Biology (A2 in a Week) (A2 in a Week): Books
Amazon.co.uk: AS In a Week: Chemistry (AS in a Week): Books
Amazon.co.uk: A2 in a Week Chemistry (A2 in a Week) (A2 in a Week): Books
Amazon.co.uk: AS In a Week: Physics (AS in a Week): Books
Amazon.co.uk: A2 in a Week Physics (A2 in a Week) (A2 in a Week): Books
For the same sort of material in more detail the Letts AS & A2 books might be useful (I used those, although chemistry was the only one I got all the way through):
Amazon.co.uk: Biology (Revise AS & A2 (Combined)): Books
Amazon.co.uk: Chemistry (Revise AS & A2 (Combined)): Books
Amazon.co.uk: Physics (Revise AS & A2 (Combined)): Books
Whilst I found that a bit of background helped I don't remember any questions from September that couldn't be answered using the information provided in the question (although I don't think I could have got through the questions so fast without some background knowledge - and GAMSAT is at least as much a test of timing as anything else).
Stephen