Aside from using this thread as a means for HowlerKarma and me to bemoan how badly our GT programs overidentify high achievers as gifted and underserve gifted kids, lol, I think that what you are needing to do is:

1) try to figure out if this program is going to be able to serve the needs of a HG kid or if it is just fluff for high achievers. I've recommended this article here before, but I think that it is important for the staff to understand the distinction: http://www.giftededucation.org.nz/documents/high-achievers-pdf.pdf
It might be worth doing a little feeling out of the teacher and other staff in the fall to see what definition of "gifted" they are operating off of. Do they understand the difference that Ms. Cathcart expounds on in that article or do they think that the A student/teacher pleasers are the most gifted?

2) try to figure out what the criteria for admission was and how many other kids fell at a similar point to your dd to see if there is any chance that she'll have true peers (perhaps from the other schools since the kids who are pulling from her current school don't seem to be peers).

3) see what other additional options exist if this program is not enough. Will they consider subject acceleration, grade skipping, or anything else?

4) look into extracurricular options for building a social base if it isn't to be had in the school GT program. We initially tried a summer camp through talent search and a summer program through a local school's STEM program for my oldest. We, honestly, found a lot of what we'd found in the public school GT program. There were some gifted kids, but by and large, the other campers were not at the same LOG and dd didn't find them to be any better of peers. We wound up finding the best fit at extracurricular activities that were geared toward kids who were much older than she and in the academic realm, but not in the area of her greatest passion so she wasn't going in with so much background knowledge that she found the material itself boring. This has worked better as dd has gotten older and doesn't stand out among older kids physically (they can't tell looking at her that she is younger and often accept her as a peer before they realize the age difference). It has also been easier since she grade skipped b/c a lot of academic programs admit by grade rather than age.

Study Strategies for Accelerated Learners