The stage 2 diet is not meant for maintenance and i don't think that it is a sustainable way of eating.

I'd recommend dropping gluten, as it's known to cause delayed gastric emptying in people with coeliacs disease and i don't think that it's impossible that even people with non-coeliac gluten sensitivity could experience the same effect. As you didn't have any damage to your intestines i'd assume you don't have coeliacs, in which case the gluten free diet should work quite quickly if it was to work, say, in 2 to 10 weeks time. If you don't see any improvement at all i'd just drop it after a few weeks, as there are no health benefits to eating gluten-free for tolerant people.

There are many causes for gastroparesis, from mitochondrial disease to connective tissue disease to autonomic dysfunction, systemic diseases like scleroderma, spinal cord injury. There is almost always a cause, it's just that it's often really hard or even impossible to establish with modern medical technology and we still don't know all that much about how the body works.

I'm not a doctor and i'm not an expert of any kind on medical things. And sorry for any incoherence, english isn't my 1st language.