Yup, what she said. Don't stress.
Find someone who knows gifted kids or knows that your DD is smarter than the average bear. As long as the person isn't going to write "This kid is NOT gifted," you're probably fine. The teachers who know the child is going to a gifted school next year are probably fine.
I suspect the letter is just to make sure some other person knows that this person is real and has a child. Seriously. I suspect it's more about confirming that we exist IRL and we're not fakers who are just trying to get access to a list of PG kids than anything else.