It isn't that I necessarily think the autism specialists will misdiagnose autism if it isn't there. I think you equally run the risk that concerns will be dismissed. Wherever you go it is can be a very tricky diagnosis. There are a lot of HG and PG kids who appear MUCH quirkier as preschoolers than they do later. I don't say this to dismiss the value of getting help because there is a lot of good that can come from it. But, I would also not overlook the very real problems of misdiagnosis.
I wasn't clear from the original poster - what are your limitations in terms of travel and expense? If you have flexibility I would urge you to see somebody who has seen many kids who are both highly gifted and on the spectrum. That's a handful of specialists but I really don't think short of seeing someone who really specializes in 2E kids that you are going to be at all confident in any diagnosis you receive - or that you don't.
