I would only consider those degrees if you are absolutely certain you want to do medicine and nothing else. And, in the case that you are unable to do medicine, are okay with backup careers being somewhere in the vicinity of scientist or researcher (this is not a conclusive list). To get into postgrad medicine, you not only need a good GAMSAT, you also need a competitive GPA (I am actually not certain of this, my post grad medicine knowledge is pretty limited) which is easier to get with a course you have an interest in, as it is very hard to get a very high GPA in a course such as medical or biomedical science where almost everyone else is working towards the same goal as you: medicine. Obviously I'm not in uni so I cannot personally attest to the competitive nature of "premed" degrees, but it's pretty common to hear people speak about it.
I suggest you chose a degree that, in the event that medicine doesn't turn out right for you or you weren't able to get in, you would be okay with pursuing a career in. There are a lot of people who apply for post grad medicine with degrees like law, teaching, or even just any random humanities, and have found the the degree in science is not necessarily a prerequisite to post grad medicine (they have bridging courses for that). I say follow what you want, there are some people who genuinely enjoy medical or biomedical science, but the large majority are only doing the course as a stepping stone to medicine, and that's on universities using these degrees to attract those premed students for money. Think about UNSW's catch in med science where the top 10% can get into med after the first year or something like that.
Best of luck!