What is the long-term outcome for patients with gastroparesis?

Depends. This depends mostly on what the cause of the gastroparesis is. If it is due to diabetes, your ultimate outcome mostly depends on how well you control your blood sugars. If you do well, your disease is unlikely to progress. Good luck.
Depends on cause. Depends on the cause related to diabetes good blood sugar control will help stop progression but not eliminate symptoms. Idiopathic may or may not get better and regular clinical follow up is needed. Post infection or symptoms after gastrointestinal infection usually gets better after several months.