I'm not going to begin to try to be superficial and say 'if you try you'll succeed' don't worry and so forth. As for this current moment, you need to honestly consider your options, under the circumstance where you do well in the GAMSAT, have a competitive GPA and do well in the interview, enough to get a CSP place in post graduate medicine and under the situation where you don't. You need alternative plans, and if you can't see any in biomedicine, perhaps you should consider Science, which offers different branches into other disciplines if you want to change from say the health sciences. Because you aren't disadvantaged both ways at least as far as the GAMSAT is concerned, you need to think about this.
The post-graduate medicine pathway is in no way easy as you already have speculated, but what you should do is really consider whether or not you want to medicine, and if so, just sit the test and hope for the best, but still leave plan B open.
Ultimately the decision is with you. There is still time. T-Rex provided a nice summary of what you need if you want to do well.