The scaled score is not his IQ score or the total score for any of the IQ index scores. Essentially, you get raw scores and then scaled scores from those raw scores. The scaled scores are dependent upon the age of the tester. The total of all of the scaled scores from each of the four indices is then used to calculate a composite score or full scale IQ score (FSIQ). 134 is likely his FSIQ, which is right at the 99th percentile.
The GAI is something that you could calculate yourself (or we could help you do) if you have all of the scaled scores for each index. The 145 includes the PRI, VCI, WMI, and PSI indices. We'd need the scaled scores for just the PRI and VCI to get a GAI and it should only be relevant if he did much worse on either the PSI (processing speed) or WMI (working memory) index.
The comprehension part where you mention he did worse than other parts is part of the VCI (verbal comprehension) index and it isn't testing what it sounds like. My dd12 did much worse on that segment as well and it, like you mention of your ds, was likely due to her being a very out of the box thinker. It may also have had something to do with intentionally being "funny" or socially deviant. Comprehension tests social comprehension (i.e. - what would you do with a letter you find on the ground addressed and stamped?). My dd seemed to enjoy saying that she was going to steal all kinds of stuff although she also may have come up with some truly unique ideas that didn't comport with the test designer's ideas. Perhaps your ds also had unique ideas.
As to the tester having no experience with gifted kids, that could have impacted the scores. Whether they came out too low is hard to say, though, but it is certainly possible. Do you need higher scores for any reason? 134 really should get him into any GT programs that he is interested in taking part in save for DYS.
As to the grade skip not being sufficient so let's not do it at all thought of the school, I'd really disagree. My one dd who is grade skipped (not the out of the box one) could easily have been doing college courses in many subjects by the time she was about 11, but the one year acceleration really was a good thing. No, it didn't put her at her readiness level in most subjects, although it was probably enough in math, her weakest subject. On the other hand, it was much better than no acceleration at all. It also helped place her in a better place socially b/c she relates better to older kids.
