I think questions like this are really just result-determined when someone looks back later on and thinks about what was the right decision and whether it was worth or not (and answers and responses to questions like this are rarely any good outside of someone just saying what worked for them specifically).
Have you done any practice exams? they might give you an indication of what sort of scores you may be looking at.
Also, you can do completely average or just slightly above average on one section and still do pretty good if your other sections are high. Also to note, you do have more than 2 opportunities since you can do the GAMSAT when you are finishing and have already finished your degree. It just would mean that you wouldn't be able to start your post-graduate course in 2023 but in 2024+.
EDIT: for your question on whether it is beneficial for a rookie taster test you will get answers of yes and no from different people. On one hand some will say that it would be good to get an idea on what you need to do on the day, where you need to be, real-exam pressure, invigilators, be more accustomed to the environment for your 2nd test whilst others would rather be completely prepared and only do it once and also save the money.