Not inherently harder or easier, but different.
GAMSAT is much more of an aptitude test, while MCAT is more an achievement test. Non-science students can take the GAMSAT with far less science prep, and can do just as well since much of the exam is about reading comprehension and reasoning skills.
In the end, it's your percentile that matters (i.e., how you do against others who take it). So in that respect, if you were to assume the pools taking either test are equally capable, then the GAMSAT is harder -- you need better than 80-85%ile to get in to the better Aussie schools.