If you feel it is important to rule in/out ASD, then yes, you would want an experienced psych who has worked with GT, ASD, and GT/ASD kids, so that the distinctions in presentation will be noticeable to him/her.
I would say that the likelihood of it being HG behaviors vs ASD behaviors is increased by your observation that the complicating-directions behavior is restricted to tasks that are mismatched in instructional level. And on repeated disbelief/clarification/confirmation of directions you've given him: I agree that this may be an attempt to fit a discordant piece of data into the data set he's previously established about the situation or the person giving the direction. In addition, it also sounds like there may be an element of a need for precision. It reminds me of childhood stories I heard about my PG sib, who did something similar when reporting observations. "Look at the car! Look at the red car. Look at the red convertible-top car. Look at the red convertible-top car. Look at the red convertible-top car under the billboard...etc." Of course, by the time you get to the end of this sequence, the car is 5 miles behind you... Kids who can see more angles, I think, are more likely to need specificity.
