Nice bump.
Your best bet would be to head to ACER:
www.acer.edu.au these are the guys that administer the UMAT and the GAMSAT.
You could also try
www.ausmedstudent.com/forum the GAMSAT section is very informative.
I doubt anyone here has much experience with the GAMSAT, but I can say it is very different to the UMAT. Read up at those two places, then come back and ask any specific questions you may still have - they're a lot easier to answer (and will make things easier for you to understand as well).