I have gastroparesis (and an extremely slow gut overall; for the record, Go-LYTEly doesn't work on me. At all. Doctors found out the hard way). I am stable now, my case is mild, and my treatment regimen is wonderful so I'm virtually asymptomatic 95% of the time, but I'll share my past experiences with you.
Imagine having a dead weight just sitting in your stomach after you eat. And imagine your stomach/esophagus being perpetually irritated, whether from vomiting or from food just sitting there for hours longer than your body intends for it. It's like living with a stomach virus that doesn't end.
I will say that I never screamed in pain from my gastroparesis. But I've been extremely uncomfortable, to the point where my quality of life has been adversely affected. I, like the previous poster, had gastritis as a result of my symptoms.
When I've received dilaudid in the past for GI-related discomfort, I've been aware that my pain has still existed, but I simply haven't cared so much that it's there as I did when the medication wore off. If it were up to me, I'd take a medication that actually treated the cause of my pain (i.e., slow motility, GI irritation), instead of an opioid that's slowing down my gut and duping my mind.
I wonder, like you, if some of the patients that you see have built up a tolerance as a result of their chronic pain. And I can't speak for all of them, but I wouldn't attribute all of their behaviors to psychologic causes. Gastroparesis can be truly miserable.