"The General Ability Sum of Scaled Scores is the sum of scaled scores for three Verbal Comprehension subtests (i.e., Vocabulary, Comprehension, and Similarities) and three Perceptual Reasoning subtests (i.e., Block Design, Matrix Reasoning, and Picture Concepts)."

When you add these subtest scores you plug them into the chart provided in the WISC Technical Report #4

http://www.ksde.org/Portals/0/Special%20Education%20Services/gifted/WISCIVTechReport4.pdf

and that's the GAI calculation.

I have read some raves about the GDC on these boards but I can only imagine how expensive it is. Do you know anyone in your area in education, especially GT education? They may know a good tester. I got the name of my tester from a GT educator at church.