Personally, I don't think that you should choose subjects that you think will benefit you in gamsat at the expense of you not enjoying it and doing badly and it having an impact on your GPA. In first year I was kind of the same in that I chose to select breadths and subjects that would benefit me with GAMSAT. For instance, I did Marketing and Management breadths to brush up on my essay writing skills for instance. But at the end of the day the GAMSAT doesn't require you to meticulously formulate a wonderful piece of writing across many days as most arts subjects will have as their major assessment components. I also chose Physics because I thought it would benefit me in GAMSAT only to get a H3 in it and then having to endure a whole summer relearning it again. Treat GAMSAT and Uni separately. Read widely to increase your awareness of greater vocabulary used in context for section 1 and to also familiarise yourself with non-every day writing styles eg. poetry. Find some gamsat prompts and practice planning and jotting down arguments and getting use to writing a cohesive and fluent piece of writing in 30 minutes. Or take a course... I think this is the best way to prepare yourself for the GAMSAT.
But this is my personal opinion. xD