Prednisone won't help GERD. Did you mean Prilosec, the proton pump inhibitor? Have you tried a few types of meds for reducing acid? Or is there really nothing wrong with the stomach???
Is your pain constant, or can you relate it to eating, especially any particular trigger foods?
Have you been tested with barium swallow for swallowing problems, weak lower esophageal sphincter (LES) and gastric reflux, and endoscopy to look at the inside of the esophagus? The barium will also show how bad the hernia is.
I'm sitting here with pretty bad chest pain (right under the sternum) after eating a very small meal because my stomach, which was previously repaired, has re-herniated (ripped the stitches) partly through the diaphragm again. My repaired LES is holding and I don't have acid burn, but I am taking Nexium to combat nausea.
A pain doc will help with general pain or the spine, but there SHOULD be something being done by the gastro doc, or he should be referring you to a really good thoracic surgeon. (Yes thoracic. Although the stomach is in the abdomen, a lot of work done is in the mediastinum, which is above the diaphragm.)