Where's your study proving the opposite? Or is your proposal to do absolutely nothing without a study first telling you to? G&T programs try to utilize the diversity of students, rather than just ignoring it or trying to will it away. In a world where things actually have to function prior to the conducting of infinite studies, that makes a lot of intuitive sense.
The programs can be good or a complete waste depending on how they are ran. The study referred to in the article pointed out that children do better in the short and long run if they are given extra opportunities to learn more on their own. That's the entire purpose of these programs to begin with, really. The same way as other Special Education programs which may or may not be well ran, but often do more for the students in them than letting them try and teach themselves.
What's the evidence that G&T programmes achieve better academic or life outcomes?