The way I like to look at it is: VCI and PRI are better measures of intellect while WMI and PSI measure more processing skills that enable the person to show that intellect. Low WMI and/or PSI do impact performance in that a child with the same VCI and PRI with comparably high WMI/PSI will probably outperform as child with the same VCI/PRI scores and lower WMI/PSI. However, whether he is truly "smarter" is probably dependent on how you define that.
GAI is, in my understanding, meant to be an alternate way of looking at composite or full scale intelligence if the child has a disability or something else standing in the way of the FSIQ showing how able he is.
FWIW, my one kiddo with lower PSI (average) had FSIQ and GAI scores that differed, but only by a few points and they were both gifted scores b/c her WMI was very high. My other one, who is 2e, had an 18 pt difference btwn GAI and FSIQ with the former being HG+ and the later MG. Whether I'd really consider her as highly gifted+ as the GAI says, I don't know. She's certainly incredibly creative and has that ability to create new information rather than spit back learned info that I'd associate with giftedness. Whether she's one in a thousand in terms of raw intellect, that I'm not sure of. Then again, neither am I!
