Hi--I'm a school psychologist and I think I can help. The GAI has absolutely nothing to do with the difference in scores between the VCI and PRI.
The FSIQ is ALWAYS obtained through the combination of all 4 indexes: VCI, PRI, PSI, and WMI--no matter how big or small the discrepancies are between them.
When a child exhibits Working Memory and/or Processing Speed Index scores that are significantly different than the VCI and PRI Index scores, then the GAI can be reported.
The GAI is ALWAYS obtained through the combination of the VCI and PRI indexes alone, leaving out the WMI and PSI scores.