I'm so sorry to hear what you're dealing with at your dd's school, and I'm
shocked at the principal's behavior. I have had ineffective principals, and unfriendly ones, but never one so directly unfriendly that I would receive a dirty look or not get a response to a greeting.

I am very direct, so I would probably schedule a meeting to talk to the principal one-on-one and say exactly what you have noticed. "I want to have a good relationship with you, and you have seemed distant or upset when I've spoken to you. How can we improve things?" Do you get the sense that s/he would deny it? Be rude in return? Refuse to meet with you?
Did you go over the principal's head to get the accommodations, or embarrass him/her in any way? Is the gifted or classroom teacher on your side?
I think it's completely unprofessional and horrible how you are being treated, but the bottom line is, you have made things better for your dd (assuming she isn't being treated poorly). Be proud of your advocacy and hang in there. With luck, the principal won't be there as long as your dd!
Theresa