Hi elainechew,
GAMSAT (Graduate Australian Medical School Admission Test) is for wanting to study MBBS in Australia via the Graduate Medical Program (GMP) which certain universities offer. Not for wanting to practice in Australia as a physician after completing MBBS in another country.
The Australian Medical Concil (AMC) has other examinations which allow overseas trained doctors to work in Australia. You may find more information regarding the test type/date/location here:
AMC - Examinations Menu
Hope this helps
DrMoon