This patient support community is for questions related to juvenile diabetes including celiac disease, depression, diabetic complications, hyperglycemia / diabetic keto-acidosis, hypoglycemia, islet cell transplantation, nutritional issues, parenting a diabetic child, pregnancy, pump therapy, school issues, and teens with diabetes.
We're not physicians here but all have experience with diabetes and are happy to share out opinions and suggestions. Learn what you can here and run the ideas by your child's doctor.
I can't tell from your posting if your daughter is being followed by an endocrinologist/diabetologist or "just" a general pediatrician. It's so important for us to have access to specialists, and if she doesn't (yet) my first suggestion is to get a referral to one. If you're already being followed by an endo, then ask for a referral to a gastro specialist who can focus in on that part of her system.
In the short term, try some home techniques ... like don't allow her to lie down/sleep/nap within 2 hours after eating. Elevate the head of her bed (not just pillows under her head) so that while she sleeps, gravity will work with the digestive system to move things down. Limit her fatty and spicey foods and begin to keep a food/symptom diary with her so you'll begin to see patterns of how she feels relative to what she's eaten and what she's been drinking.
Many folks with Type 1 diabetes have or develop other issues. While our disease *is* an autoimmune disease, it is not immune "deficiency" that causes our disease. In fact, our immune system is hyper (overly) sensitive and that's what's believed to cause our immune system to have attacked our beta cells in the pancreas.
Among the issues that diabetics often face is a sensitivity to wheat/gluten and/or dairy foods. While some folks have a sensitivity or allergy to either/both of these foods, other folks have a medical condition (read about Celiac and Crohns, fore example). A typical American diet is loaded with both, and you might try eliminating those foods for a while to see if her digestive system begins to work better. Is your daughter drinking plenty of plain water or other non-sugary, non-carbonated, non-caffeinated beverages? That's a good habit to get her into.
My understanding is that gastroparesis is not evaluated by doing a blood test, but rather it's done by doing a gastric emptying test(s) and perhaps some other techniques that allow the docs to see what the digestive system is doing.
Hope this helps some and I hope you'll hear from others, too.