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Great questions that I am sure many people have.
I will copy/paste your questions for ease of answering.
To apply for the Doctor of Medicine you only need to the GAMSAT and you're GPA. Correct - your GAMSAT/GPA will be used to determine whether you qualify for an interview - then your GAMSAT/GPA/INTERVIEW are used to offer places in the course.
So let's say I choose to do an arts degree, as long as I do the GAMSAT I can still apply for Medicine? Correct - You can do any bachelors level undergraduate degree - this has actually been the norm for most universities around Australia - unimelb are kind of playing catchup
Wouldn't I still be at a huge disadvantage, because the GAMSAT is heavily focused on science? This is debatable - Firstly the GAMSAT does have a science section however unimelb actually weights it the least amongst universities (one third) - a few other universities do this as well whilst many other universities give it a 50% weighting. So whilst a science background would help for the science section there are two other sections - writing and humanities which an arts degree would be helpful with.
The debatable part is whether the science background actually helps with the science section - some people say that it is useful for a baseline knowledge which means less confusion when reading questions which helps with time but doesn't help too much in answering questions. This section of the GAMSAT has a lot of logical reasoning and problem solving which just having science knowledge soley is not enough to answer - you need the ability to think, not just the knowledge.