Betty, I was dx'ed with GERD well over a decade ago. I am on the lifetime Nexium plan now, as I have Barrett's esophagus. More recently they've added Zantac on top of the Nexium, as I've been having some heavy duty gastritis.
I was also dx'ed with IBS by my GP -- but the GI doc isn't sure it's that, since I do not respond to Bentyl (it makes things way worse, not better). SIBO and Celiac have been ruled out. I can tell you that boosting my Vitamin D to 5000 daily does make a difference, so that is definitely worth a try.
I recently was in the ER with a ruptured ovarian cyst, so now my personal suspicion is that my "IBS" symptoms have been related to PCOS, which I was dx'ed with by ultrasound years ago... around same time the IBS symptoms all started. If your symptoms are at all correlated with a monthly cycle, this is something to consider.
I've tried every possible diet out there.. including FODMAP. I've never found any correlation whatsoever to diet. Your mileage may vary; it is a good place to start, as it cannot hurt. I will say I felt best eating a daily dose of plain yogurt (as in 1-2 cups of yogurt). But even then, I was still having problems, just a little less.