I believe the GT center for Greenbriar East is Greenbriar West.
I don't know much about either school as we are closer to DC, but if you look at the test scores, Greenbriar West has significantly better scores (partially because of the GT population) and it also offers more GT related extra curricular activities like chess club, Odyssey of the Mind, etc. However, it looks like Greenbriar East does offer what they call "compacted math" which is similar to the math curriculum used in the GT center. It allows kids to move at a quicker pace than the general curriculum.
It is not uncommon for kids with challenges to be enrolled in the GT program. The GT center classes tend to have a wide variety of kids, each with their own strengths, weaknesses and quirks and they all seem to connect well. The curriculum is advanced and at a quicker pace, and they further differentiate by grouping the kids according to level (at most center schools).
I have found it to be a great program and it has really been wonderful for my dd. The greatest benefit has been that she has really "found her people" at the center. At the center, it's not "weird" to be obsessed with something that other kids haven't even heard of and it's not "geeky" to be crazy smart.
They're a great bunch of kids.
Here are the school profiles
Greenbriar East:
http://commweb.fcps.edu/schoolprofile/profile.cfm?profile_id=254Greenbriar West:
http://commweb.fcps.edu/schoolprofile/profile.cfm?profile_id=255You also might want to sign up for the yahoo group at fcag.org .
It's a great place to ask questions of local gt parents.
I hope that helps!