I didn't know there's a GAI score for the WISC-IV because we were only presented with the FSIQ score plus the 4 scores, with the composite profile. I wanted to know more and found the Tech Report#4 mentioned by Dottie. http://pearsonassess.com/NR/rdonlyres/91DACD36-4E78-4F5C-976D-D6048BD67ED7/0/WISCIVTechReport4.pdf
On page 3, it talked abt when to use the GAI. Not sure if it will be helpful to you.