In terms of the GAI, the NAGC has a position paper on using the WISC in terms of identifying children for gifted programs. I turned this into the school with my DD's report, since her GAI was a lot higher than FSIQ. In her case, the coding score really brought her FSIQ down a lot. We do see this "slowness" reflected in school and it is meaningful, but most psychs would say that kids should be put at the right level in terms of reasoning ability, and be given supports for weaknesses in other areas (like speed). Some gifted kids are very careful and deep with their thinking and that slows them down. I don't think this should be viewed as a "problem" but it can seem like a problem in terms of lower scores. In other kids it represents a disability, like ADHD or poor motor skills. My DS's coding score was low because when he was tested his ability to write was abysmal. The same thing happened with block design, which is timed and involves motor skills. Some psychs would have deliberately substituted a different test, but the goal wasn't to find out if he was gifted, it was to identify problems due to a brain injury and delays.
http://www.nagc.org/index.aspx?id=2455
