Final answer:
Gregory's experience is most accurately characterized as a somatic symptom disorder.
Step-by-step explanation:
Based on the symptoms described, Gregory's experience is most accurately characterized as a somatic symptom disorder. Somatic symptom disorder is a condition in which a person experiences physical symptoms, such as digestive issues, sleep problems, and pain, without a clear medical cause. It is common for individuals with somatic symptom disorder to attribute their physical symptoms to a medical condition, despite the lack of evidence supporting such a diagnosis.