I'm so sorry to hear that your DGD has been dx w/ gastroparesis. I know it must be so hard to watch your DD and your DGD both suffer w/ their illnesses.
My mother and my husband were both dx w/ gastroparesis. My DH was dx a cpl of yrs ago, after many different hospital visits for several illnesses. I had asked if this might be the case and his dr wasn't too sure, TILL he did an endoscopy and the 'egg' test. My DH is also a Diabetic, and this is a common condition for diabetics, but ppl w/o diabetes can also develope this condition.
When my DM was first dx, there was a lady on the Fibro forum that had gastroparesis. She gave me some ideas and also suggested I check the Mayo Clinic.com site. I found some good info on that site.
They mention changing your diet and some meds that may be given to ppl. One med suggested, Reglan, is what my DH and I take for different conditions. Many gastros do not like to prescribe this med, as it can cause some other problems. For my DH and me, the other meds do not control the nausea or vomiting. We use this med only when absolutely necessary. One thing that helps my DH for both diabetes and gastroparesis is smaller meals, you can also use liquid meals - such as Ensure. I would suggest your DD ask your DGDs drs for a visit or two to a visit w/ a registered dietitian. S/he should be able to give some solid information on how and what would work best for your DGD.
I wish you and your family the best.
God bless. Alice.