It's hard to add to aeh's wealth of wisdom, but a couple of quick thoughts from the parent of two teens with inattentive ADHD (and a cornucopia of other Es smile ).

If you can, find a psychologist with real experience of highly-gifted teens. Every provider of every kind I ever talked to all assured me, "Of course I've worked with tons of gifted kids". No, no they hadn't. They'd works with tons of bright kids, that top 20th percentile who are always at the top of the class. Different group, different need - and totally different compensation skills. So I recommend asking some very specific questions about the kinds of HG kids they've worked with, how they found them different to assess, and what they did differently to tease out the challenges in the face of such powerful ability to hide them. If they can't articulate any of this, I'd guess they don't have actual HG experience.

As aeh notes, there are a lot of things that can affect attention and look like inattentive ADHD, including especially anxiety, but also auditory processing. And the impact of other LDs being increasingly hard to compensate for can also trigger existing ADHD, making it suddenly become a much bigger problem and much harder to manage.

Originally Posted by MorningStar
If she is able to sit through ACT test, pull 36 on it and move on without meltdown (and PSAT testings) should this be a sign that she is OK?

Nope. My elder (17) is the more extreme for both gifted and ADHD. It's extraordinary what he can pull off - and what he can't. Inconsistency is the core of the ADHD beast, and it's very confusing as a parent or a teacher. But surely if you did this, then you could easily do that?!!" No, they actually can't, they really just can't. The level of effort required can be achieved when the stars and planets are in perfect alignment and you throw in some extremely high intrinsic motivation for this specific task, but this is not sustainable.

I found Russell Barkley immensely helpful for understanding the particular weirdnesses of inattentive ADHD; for instance: http://www.russellbarkley.org/factsheets/WhatCausesADHD2017.pdf

Originally Posted by MorningStar
My husband do have undiagnosed ADHD (we don’t have to test him for it, anyone can see it :)), so definitely family history there too

Final thought - any chance your husband's ADHD is of the more visible, externally-manifested hyperactive/impulsive type? Because that will look extremely different from inattentive, and could be contributing to your confusion. I didn't really even know the inattentive kind existed before I started this journey, so it took me a long while to wrap my head around the idea that my kids had ADHD, despite looking so utterly different from friends and family with ADHD.

Best of luck to you. I hope the assessment goes well and your DD finds the support she's looking for.