Like others here, I don't exactly advise studying for the GAT, but I do advise taking a look at the GAT paper so that you are comfortable with it, and that is would be no surprises to you when you come to doing the GAT as you'll know what to expect.
Also, familiarise yourself with the writing tasks. Hopefully, they picked a good topic, that you are willing to discuss. Last year, I had to do the GAT and didn't like one of the topics (it was about ceremonies... So boring). So I was pretty much bored for the whole half hour and written what basically entails a complaint about boring assemblies at school  

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Aside. Don't try in the GAT, you lose your insurance in case anything happens to you.  
