I was disappointed, to be honest. Not a lot of 'meat' in the article. I was hoping for a more specific analysis of conformation, locomotion, etc. but there were no specifics at all about what she looks for, just some general platitudes about how she develops them. Maybe she's not willing to give away her secrets just yet.
"You become responsible, forever, for what you have tamed." - The Little Prince
Well ... retrofit is right, in that she says "pick good hind leg conformation." If they don't have a big motor with correct, strong joints, it's never going to happen -- she says "healthy" but I think she means no joint issues, things like that.
She says they should have more than three good gaits, too.
And even she sometimes can't tell until the horse is 6, even if she's been riding them since they were four, although that helps her decide earlier.
So, I think the bottom line is, it's that work ethic, spark, and drive that makes the GP horse stand out from the others, given all other factors such as conformation are equal.
The only way to determine that is to ride them, and see their attitude and soundness.