I thought there needs to be specific point spread (maybe 23 pts?) between the highest and lowest scores in order to use the GAI. Let me see if I can find it... here, the full scale score must not be used if the point spread is 23 or more, but it doesn't say not to calculate the GAI if the point spread is less, at least I don't see it here (I'm glad because that wouldn't make sense). Other interesting tidbits at this link too...

http://www.gifteddevelopment.com/About_GDC/newiqtests.htm

Originally Posted by GDC, again
When there are extreme discrepancies between composite scores on the WISC-IV (23 points or more), the Full Scale IQ score should not be derived (Flanagan & Kaufman, 2004). Therefore, the Full Scale IQ score should not be the main score used to determine program selection (Rimm, Gilman & Silverman, in press).
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When using the WISC-IV, either the General Ability Index (GAI), which emphasizes reasoning ability, or the Full Scale IQ Score (FSIQ), should be acceptable for selection to gifted programs. The GAI should be derived using the table provided by Harcourt Assessments (Technical Report 4). The Verbal Comprehension Index (VCI) or the Perceptual Reasoning Index (PRI) are also independently appropriate for selection to programs for the gifted, especially for culturally diverse, bilingual, twice-exceptional students or visual-spatial learners. It is important that a good match be made between the strengths of the child and the attributes of the program. Students who have special learning needs should be admitted to gifted programs, provided that there are other indications of giftedness and instructional modifications are made to fit the needs of the students (NAGC Task Force on Assessment, 2006).
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The gifted validation sample reported in the Technical Manual of the WISC-IV achieved a mean Full Scale IQ score of 123.5 (Wechsler, 2003). The mean IQ score of 202 children in the gifted validation sample of the SB5 was 124. Therefore, cut-off scores for gifted programs should be lowered to 120, rather than 130 (Rimm, Gilman & Silverman, in press; Silverman, in press, a).
I think Silverman's argument is interesting - I don't know if she says it quite this way, but it seems like she's saying, either use the GAI or lower the bar for admission to a gifted program to 120 for full-scale IQ.