I'd say GAMSAT isn't the most important thing in the world. GPA I would rate much more highly.
You only need the GAMSAT to get into a select bunch of postgraduate degrees. You need a good GPA (or at least passes) to get into most all postgraduate degrees. And often, for internships or employment.
That said you can use your breadth to improve your skills. I would take a subject which involves some writing (ie most any uni breadth subject or Arts type subject, many commerce subjects) to practice your skills, but in a topic which interests you. For instance, I took a criminology subject with a focus on biological causes of crime last semester. It interested me because biology. It also reintroduced me to essay writing, which I guess would be helpful for the writing component of the GAMSAT, among other things, especially since most of the examinations were 30 minute essays. If you like history, do some history. If you're interested in marketing, do some marketing. I'm sure any subject that interests you will let you practice your writing skills.