Ironically, one of the most effective things I've found against the GP was Erythromycin. My GI doc had me try it (after I refused the Reglan!) and it worked for about a month or so and then the effect wore off. It actually worked a little too well (pushed food through too fast).
My health problems started many years ago and gradually built up until the GP started in suddenly just over two years ago. Only more recently have I had Antibiotics (and a lot of them, for treatment of suspected Lyme), but they haven't had any noticeable effect either way on the GP.
But, after going through a long period of depression, I am working hard on this. With some further dietary changes, some herbals, and lots of prayer, I have hope of turning the corner at some point soon.