If she is literally not consuming any solid food in a 24 hour period, then yes, I'd be concerned too and I'm surprised that your GP was not. I'd be seeking a second opinion and asking for a referral to a pediatric occupational therapist or speech pathologist to assess why she is refusing food.

You will likely be encouraged to refuse her milk in the hope that she will be hungry enough to try other things, but if her intake is as limited as you describe then I would only do this under the supervision of a medical professional.