My understanding (which may be incorrect), is that school scaling can pull your marks down noticeably. So if a teacher marks easier than other schools and all of the students get As during the year, but then the students get Bs in the exam, VCAA thinks "hey, the school was marking easy!" and scales down the SAC results using the exam results (everyone got the same exam and same marking regime; SACs and their marking vary wildly, so exam performance trumps SAC performance). This is what often leads students who felt that they were going well during the year to get a worse end of year result (note that if you personally were doing great, but your classmates weren't, you'll get scaled down with the rest of them). The technical report on school scaling is pretty hard to grasp (and I have a degree in stats.).
And no I don't think that you could use the GAT, it more exists as a fallback in case you missed the exam or something.