Science actually helps you prepare for the GAMSAT better than Biomed. They leave out stuff in Biomed because we cram chemistry and physics for one semester each, whereas science you do it for like a whole year or something.
Also, a lot of the GAMSAT is focused on scientific reasoning and much less so on your ability to recall knowledge. A good foundation is useful and will help your timing, but a lot of the material will be on things you've never studied so you need to get into the habit of critically thinking about all the things you learn and how to creatively apply it in new situations, simply by doing more and more questions from anything you can get your hands on. Generally speaking they give you enough information in the question stem for you to work with, it's up to you to apply it.
A better forum to ask about GAMSAT is pagingdr.net. A lot of people there also have no science backgrounds so it's worth checking it out.