What is a safe bowel prep for a severe GERD patient on a high dose of PPIs? Moviprep, Suprep, Propopik and PEG/Mag Citrate all contain harsh acids.

Bowel prep. Your gastroenterologist is best qualified to answer your question, but I don't feel that any of the recommended preps would be a significant problem for your esophagus while you're on ppi therapy. My guess is that the gastroenterologist has a preference for a good bowel prep and will prescribe it.
All safe. None of these contain the type of "harsh acid" which would be expected to worsen gastritis or GERD. So long as you continue your PPI, you should be able to complete any of these preps without significantly worsening your GERD symptoms. These products have an excellent track record of safety and efficacy, even in people with ulcers, gastritis, and reflux. Good luck!