The gamsat only really requires you to Know first year chem, so you can't go wrong with the chem 1 - chem 2. If you want you, can still do chem for bio science cause it still gives you adequate prep for gamsat, but the subject just requires more effort. I didn't do chem for bio science, I did biomed, but if you think you are good enough to do chem for bio science, you good enough to "cruise chem 1-chem 2" (using the word (loosely). This is simply because chem for bio science is just chem 2 with a "bit" of focus on biological things, like ur forced to remember the structure haemoglobin. The decision is up to you. All that you need to consider is whether you feel like you need the basics (if not just do chem for bio science) or simply cant be bothered to do two chemistry units in first year. However, in my opinion, doing chem1-chem2 just seems better because you have more time to digest concepts.