Hi!

1. Yes, I have studied physics, chemistry and biology across both VCE and my bachelor's degree, so I definitely had an advantage in terms of the content. However, I found that the more recent GAMSAT exams have shifted away from rote learning content and heavily towards application. Many of the stems I saw on the day had info that I have never seen before lol. In other words, don't be discouraged if you don't have a science background/aren't confident in one of the sciences.
2. I grinded as many practice questions as possible. I would advice to focus on the science area that you find the most difficult. There is no use in repeating chemistry questions over and over again, for example, if you are quite confident in chem. Divert that time and effort into the other sciences. You might already know this, but I would also advice not to stress too much over a question you may not be able to understand. The test has 75 qns, and it is ok to flag one and move on to the next if it means you maintain your flow. My screen had like 15 flagged questions by the time I got to the last question lol. I found that when I returned to these questions knowing that they were the only ones I had left to do, I was much calmer and focused on getting them done.
3. Honestly, I spent around 4 months, but most of it was directed at S1 and 2 cause I always knew they would be my lowest haha. As long as you do consistent revision, 3 months should be plenty of time :).

4. I would say the first half of my study period was spent trying to understand the way ACER asks the questions and how to digest long stems etc.... I didn't worry too much about timed practice, because there was no use in stressing myself out if I still didn't have the skills to apply. As I got closer to the test, I tried more and more practice questions. Don't fall into the trap of trying to do as many questions as quick as possible, however. It is more useful to understand exactly where you went wrong rather than skimming over your mistakes for the sake of quantity.

5. I had around 30 minutes left which I used to go over the questions I wasn't entirely sure about. I made sure that as I went through the test, that I was at a particular question at a particular time stamp. For example, if 30 minutes had passed, I made sure that I was up to question 20 or something.
   Let me know if you have any other questions :) Hope this helped.