Yeah, we didn't have the GDC test my younger dd, just review other tests she had taken and give us her opinion as to what was going on (a consult). It wasn't superbly expensive, but I also found it to be unhelpful and the woman with whom we consulted was rude to me and rather condesending. As I said, I wasn't too impressed with her and didn't walk away believing all of what she said (and this wasn't due to her think that dd was not gifted, she pretty clearly stated that she thought that dd was quite gifted).
I asked about the sibling spread info from their webpage that samson11 quoted above. She said that those instances where they found large spreads (the 1/4 with greater than 13 pt differences as quoted above), were where one sibling had a PG IQ and another had a MG IQ -- that they hadn't found any 130/115 siblings for instance, but they had found 150/132 siblings.
