I am having a crises with this. I am seriously considering a gap year + repeating the UMAT right now.
The GAMSAT essays look terrifying, and I plain stink at a lot of that section 1 stuff.
You can sit the GAMSAT in the second last and last year of your undergrad degree, which is a benefit.
I don't believe there is a limit to the number of times you can sit the GAMSAT, however I am under the impression that results are only valid for 2 years.
As for what you said last, this is my fear also, which is why I am considering the gap year, so I can remain eligible for Monash.
You will probably need to do honours and other post grad studies to get a decent job with biomed. Or you can opt for other health related degrees such as optometry, dent, physio etc.