Gastroparesis can be caused by autonomic neuropathy, but I'd think that usually you'd have other symptoms of autonomic neuropathy as well. Your autonomic nervous system controls more than just digestion, it controls basically everything your body does unconsciously (like heart rate, blood pressure, pupil dilation, etc). This can make you dizzy, have exercise intolerance, sweating problems...
Like phtgrapher said, gastroparesis can be idiopathic. I wouldn't really worry about autonomic neuropathy. The treatment for it just consists of controlling symptoms...so it would just be controlling the gastroparesis which I'm assuming you're already doing. Causes such as diabetes are good to know and helpful cause you can treat it. I would just make sure to rule out those causes...
I'm sure you've done some research into gastroparesis. As you may or may not know, it can be caused by lots of things.
Medicines that slow down the gut like anticholinergics(tricyclic antidepressants, etc).
It can be caused by diabetes.
Damage to the vagus nerve which comes from the brain directly to the stomach. Which is usually damaged by surgery to the stomach abdominal region.
I had a chiropractor once say that gastroparesis and excess acid, stomach upset (which I don't know if you have) can be caused by misalignment and irritation of the nerves that come out of the back to the stomach. They say that nerves control everything in the body and if you have problems there it may become physical.
It can also be idiopathic which is no real known cause.
Autonomic reasons may be feasible as to a cause.
For myself I have be dealing with stuff for so long and tried different stuff I am going to try acupuncture. I'll let you know how it works.