There are definitely some bizarre myths that surround the GAT and its purpose.
Firstly, the GAT does NOT directly impact on your SAC scores. VCAA vaguely claim that the GAT is used for statistical moderation in some subjects but generally speaking, it isn't used unless GAT scores are deemed more reliable than examination scores.
The GAT is also used to determine which students' VCE exams will be marked a second time (this is only applicable to subjects in which exams are only marked once- accounting, maths and sciences). VCAA states that, "a student’s GAT score is used to calculate their predicted examination score. If a student’s examination score is significantly different from this predicted score, their paper will be marked again" (VCAA website)
The GAT is also used for the calculation of a derived exam score.
Essentially the GAT is something that you shouldn't really lose any sleep over, as it can only impact on you positively. It's simply used as a predictive tool which has the capacity to improve the validity of your VCE results.
You can read more on how the GAT is used at the following link:
http://www.vcaa.vic.edu.au/vce/exams/gat/relates.html