As a child I was extremely shy and a selective mute in certain situations. It didn’t help that I had an extremely confident and gregarious mother who tried to persuade, cajole, encourage me to speak to people in all sorts of situations. I remember the feeling, the more she pushed, of being simply unable to get any words out of my mouth. I think she often interpreted it as me ‘being awkward’ but it was the most horrible feeling and completely beyond my control. I gradually grew out of it but I never lost my shyness until much later. It reared its horrible head again on one occasion in secondary school when a teacher asked me to read something - for some reason I had the same helpless feeling and the more I tried the more I felt unable to form any words. The rest of the class thought I was being deliberately insolent, and actually saw me as some kind of perverse hero (it was a very strict school). To this day my toes curl with embarrassment when I remember that incident. .
Please be kind and gentle with your DGD. This is not within her control, she is not doing it deliberately to wind anyone up, and she is probably very unhappy about it. If all else seems well I’m sure she will grow out of it in her own time, but this will happen more easily if the adults around her do not put pressure on her and do not draw attention to it.. I feel for her.