Are trials for NSW like the GAT for VCE?
I don't really know what the GAT is, except for miniturtle's answer, but I can explain what trials are

Throughout the year we have about 4 internal exams for each subject which together make up 50% of our final HSC mark. Trials are the final internal exams which are set out the same as our external HSC exams, and they are worth the largest percentage of our internal mark (usually around 35%). Because they are set out the same, they kind of give an indication of whether you are on track/how you will go in the final HSC exams and what still needs work. They are also important because they are the final chance to improve your rank, which influences your internal mark. This is kind of confusing, hope it makes sense!