Just in case you havn't come across this report in your search. The WISC–IV Technical Report #4 has info about the GAI
"This report provides information about the derivation and
uses of the General Ability Index (GAI). The GAI is a composite
score that is based on 3 Verbal Comprehension and 3
Perceptual Reasoning subtests, and does not include the
Working Memory or Processing Speed subtests included in the
Full Scale IQ (FSIQ)."
http://www.ksde.org/Portals/0/Special%20Education%20Services/gifted/WISCIVTechReport4.pdf
