Hi there,
I am un-sure whether you are asking "what does the GAT mark equate to a Study Score"
However, the GAT is not designed to provide a mark to resemble your Study Score in a particular subject.
The VCAA states that students who perform well in the GAT tend to perform well in VCE, but thats just as far as it goes, because as you pointed out not many people take the GAT seriously and many of the questions are irrelavent to particluar subjects.
In addition there are MANY people who did extremely poor in the GAT, but did not have this represented by their excellent Study Score.
Albeit, for me the GAT was able to identify my weakness in Science and strength in Written Communication.
Hope that answers your question.