Thanks for the support everyone. My sisters have children, and while my nieces and nephews are all smart kids, none of them are GT. I was also in that category growing up - a very high performer, but I worked hard for my grades and it wasn't something that just came to me, and I know I wasn't doing things like DD, which is figuring out multiplication on her own in 1st & 2nd grades before it's been introduced by the school or taught to her by us.
It is a good point to try to document her triggers. Sometimes, speaking with her after the fact, I can weasel the trigger out of her; other times she will refuse to talk about anything that caused a meltdown, even days later and will have a meltdown even thinking of the past meltdown. It's nice when the triggers are obvious like uncomfortable clothing or foods she dislikes or loud noise, but so many of them are not clear to the ordinary person although of course, DD thinks anyone should be upset by whatever happened (or the reason for the trigger is convoluted).
In kindergarten, DD would not play with other kids at all at recess or even on the play equipment, but starting last year, she has made 2 good friends she always plays with at recess (though she gets distraught if the 2 girls have a disagreement on how/what they will play during recess). Her friends are both exceptionally outgoing girls, and one of the little girls is extremely wonderful with DD, helping her with deep breathing to calm down and seems to understand that DD is different from other kids and that is just the way DD is.
I am lucky DD has had very supportive teachers at this school and of course, I don't think her principal could do a better job. I'm also happy we got her into the dual language program (it is only offered at one elementary school in the district, and we had to apply then re-district because DD's school is not her assigned elementary school). I think if she didn't have to learn Spanish half the day, she would be bored in school, but I don't think she is emotionally equipped to skip ahead to higher grades, never mind if she is reading English at nearly 6th grade level and usually has straight 100s in math. Still, I can imagine how exhausting it has to be for her teachers knowing DD will likely have at least 1 meltdown a day and having no idea what might trigger it.