Agree with the above, it's better to follow your interests than to pick subjects "strategically".
No major is going to be really helpful for getting
into medicine since admissions treats all applicants equally, as long as you've done the Anatomy, Physiology and Biochemistry pre-reqs in 2nd year you're good to apply.
As for what majors would give you an edge up when you actually study medicine, it is said they only help a little bit in first year med because they'll quickly revise it. It's pretty much the same advantage people who studied Year 12 bio get when they study first year bio...it helps but in the end it hardly makes a difference.
What are the subjects I would need to undertake if I wish to do medicine? I am enrolled in a bachelor of science at Melbourne. I didn't do physics in year 12 so I'm considering taking that up because it is assessed in the gamsat. Also, I loved methods in year 12, so I'm thinking of doing that aswell. Help please!
Did you do Specialist? If you did then you'll probably do Calculus 2 if you want to keep doing maths. If you didn't, it is said Calculus 1 covers spesh pretty well. Obviously not necessary for medicine but if you're interested you can schedule it in your study plan.
Studying physics does help for the GAMSAT but quite frankly it has a poor reputation (look a the subject reviews). And you can always self-study it. If you however work hard and persist, doing it at uni is probably the better option.