This is purely anecdotal, based on our personal experience, but IMO the problem with the GT program is there is absolutely NO continuity among school districts. That I in ISD means what it stands for when it comes to GT.
My son tested into the GT program in CSISD in K and officially started the program in 1st grade. He LOVED it, and I was thrilled with the quality of the GT certified teachers who he was assigned to for "regular" class. In the middle of 2nd grade, we had to move out of CSISD. For a few weeks, I was unsure exactly where/what ISD we would end up. I called around to several potential programs, and that's when it became clear just how different the programs are. In particular, the tests that are administered and the scores required for admission to the program vary widely from district to district. The other districts had absolutely no interest in my son's GT scores from CSISD. I was really interested in one district that actually has a special middle school just for GT students, but, ultimately, that's not the area of town/district where we wound up.
When we finally settled on a new school, he was not allowed to transition into their GT program to finish out the school year (we moved in April, so it would have only been 2 months). They did re-test him (using their methods) and, of course, he did not place in GT for 3rd grade. Honestly, I took it harder than he did. Despite not being in GT, he performed wonderfully in 3rd grade...maybe even better than in previous years. He was still being taught by GT-certified teachers in the "regular" classes, and I think that made a huge difference (and was my biggest concern when he did not place in GT). He was tested again at the end of 3rd grade and is now back in the GT program for 4th grade.
As much as he loves the program, I don't think he would be missing out on anything if he wasn't in it. He loves the smaller class size, ability to interact with other kids who have similar interests/learning styles as he does, and he likes the special projects that they do. But, he is still highly motivated to do well in his "regular" classes. I have been very pleased with all of the GT certified teachers that have taught him in 1st-4th grade, so I think that does make a difference...if for no other reason that the GT certified teachers now how to manage a GT child in the regular classroom.
As a side note, the GT teacher/counselor at our current school said that there is a push in our district to do away with the pull-out GT program altogether and just have GT-certified teachers in every classroom. She said they would be tightening the scores required for admission into the program in order to reduce the number of GT students. She said part of the reason for this is the lack of state funding for GT-type programs. Take all of that with a grain of salt, though...I never followed up to verify the accuracy of what she told me.