I'm thinking...
800-101 Critical Thinking With Data
800-100 Seeing: The Whole Picture
I did "seeing: the whole picture" as breadth. I found some sections to be very boring, however, this will differ according to your interests. There's a bit of science content (evolution of eyes, overview of eye design...), which you might find interesting.
Not sure whether it would prepare you for the GAMSAT. I guess the multi-disciplinary nature of the subject teaches you to tackle an issue from many different angles (though, this also applies to many other subjects). The essay would be good practice for the writing task. You are expected to write a fluent, thought-provoking and well structured essay. Bear in mind, however, that it is the main form of assessment, so you need to be quite comfortable with writing essays
