I would say that if you do Science/Biomed, then you should be fine for Section 3 GAMSAT. You will just have to learn a little bit more about chem and go through some VCE Physics. The Biology is more or less graph reading than anything else.
As for Section 2, it is similar to the Writing Tasks on the GAT (which is funny because ACER also make the GAT). You get presented with quotes on two topics and then you have to write two essays in an hour. With practice, I don't envisage this section being too difficult, but it requires a lot of discipline and effort.
Section 1 is basically humanities. It involves analysis of fictional/non fictional texts, poetry, proverbs & quotes, cartoons, diagrams, pictures, tables and charts. Then you have to answer MC questions on them. It is very wordy.
You can sit GAMSAT 4 times before applying for med (September 1st Year, March 2nd year, September 2nd year, March 3rd year) so you get a decent number of goes at it. Also, you can take an honours year to boost your GPA and take GAMSAT another two times if you get desperate.
And Biomed is designed to be either a precursor for professional post grad health science related degrees (i.e. med/dent/physio/pharmacy/optom/vet etc.) or for higher studies and research. You may be able to get a job with just a biomed degree though.
If you want a broader set of skills and higher chance of employment after your undergrad in case you don't get in, then I would go with science.