One possible reason why the G&T topic is so vexing:
There are a few forms of talent where very exceptional ability can be possessed at an early age and might call for some sort of special eductational intervention. I'm thinking of mathematical/musical precociousness (chess-playing skills might fall into htis category I guess)
But for the vast majority of manifestations of intelligence, the intelligence is not something that exists on a single dimension, like height, or weight, or whatever. It is a complex involving narrow features like IQ but also much broader things like personality, emotion, character, maturity. As such, it can't be separated from the person as a whole, and can't be perceived except across the child's whole life and experience.
Parents know their own children well and see this growing intelligence like no outsider can, and so they might tend to feel that a child who is singled out on the basis of narrower criteria is unreasonably being judged 'better' than their own child.