I used a range, but I admit I panicked and paid for GradReady. They have free resources, but I mainly wanted to use the question banks, as you can do loads of practice and monitor your progress. They also have study materials.
I also used the free materials available from the Acer GAMSAT site.
I have a pretty good science grounding, but used free materials on
https://www.masterorganicchemistry.com to boost my chemistry.
For section 2, I read a lot of Ted talks, as these are short, cover a lot of topics that are likely to come up and give you examples you can use. I prefer reading to watching videos, so I just read the transcripts, as it's quicker and more to my liking. I also went through all the past papers I could find and made a list of topics that came up and, as far as possible, noted some possible examples I could use for an argumentative essay (from literature I've already read, from broadsheet news sites etc. To be honest, anything can be an example, you just want a good bank of them to use). I then did some practice typing essays in half an hour. What helped me most here, was realising that your essay is unlikely to be good, i.e. to a standard you would usually be happy with, it just needs to be good enough. Therefore, structure and examples to support your arguments are important. Of course, you might not want to do an argumentative essay, you might want to do something more creative, but I know I could not think about that in the time given (I know my limits!).
I would look through this thread, as other people will have made recommendations too.
Further, I found one of the hardest parts was being able to eat sufficiently and have enough access to water during the whole exam process. I planned my food, took lots with me to eat before and in the break, but due to delays and stress, I was already shaking with low blood sugar by the time it started (though exam details said 2pm start, they only started checking people in then - my exam finished at 8:40pm). Make sure you take plenty of water with you in case there is nowhere to fill up your bottle/purchase more (if it's in a hotel, ask at the bar to refill your bottle - may seem obvious, but this kind of thing was not signposted in any way where I did my exam and I only realised in the break). I believe the break has been increased from 30 mins to 1 hr, though.
Hope that helps. Good luck with it.