Did the tester comment as to whether his block design score was impacted by his processing speed issues? That test is timed and he was obviously very good at it, possibly better at it than shown due to the processing speed issue.

I can't help with GAI but my understanding is that this is a the classic scenario where it is relevant - his PRI and VCI are in the same vicinity, his WMI is not uncommonly lower for a gifted child but the PSI is clearly a problem area making the FSIQ invalid, I am surprised it was calculated at all.