Welcome cmac! I think you are very lucky that your DD tested into the GT program without having to go through some sort of nomination process where she may have been overlooked because she doesn't exhibit "typical" GT behavior. Although from what you do say, she sounds plenty GT (advanced vocab, reading above grade level, performing well in school). Not all GT kids look crazy gifted, maybe not even most GT kids. It depends on personality and other factors.
My DS will occasionally voraciously read a book (eg., the Hawking kids' books about George), but usually he'll just read the recommended 30 minutes the teacher wants him to read daily. He is friends with everyone, but doesn't seem to have a best friend (though he did seem to make friends more quickly when he went to a FT GT program). Before the GT school, he wouldn't complain about school much, but we knew when we saw the work that he was doing that he already mastered it years earlier. Once we transferred him, he complained that he no longer had time to daydream, apparently his favorite activity in the regular school.
My thoughts were always that I would have to work harder to get our DS appropriate level work at school, since he would happily fly under the radar and keep his daydream time if he could. I think girls may be in greater danger of not making waves and being teacher pleasers. Obviously, I'm quite biased, but I'd say go for the GT school, so your DD can learn how to learn new stuff, like everyone else already has the opportunity to do. It sounds like she would fit in anywhere.