"Why is gluten ok on a low-FODMAP diet? I thought gluten was bad for people with IBS?"
The answer is very simple. Gluten is not a FODMAP. One of the most persistent myths about low-FODMAP diets is that the diet must be gluten-free. Not true. It's an understandable mistake, because wheat, barley and rye are dietary sources of both gluten and FODMAPs. Gluten is a protein found in these grains. FODMAPs, on the other hand, are carbohydrates, in this case specific types of fiber called oligosaccharides.
Is gluten a potential cause of GI symptoms? Yes. And yes, it's fairly easy to do a gluten-free version of an elimination diet. Yes, some gluten free foods are suitable for low FODMAP diets. But those facts are all a little beside the point.
It's important to remember what a FODMAP elimination diet isn't and what it is. It is not "an all-purpose , one-size fits all diet eliminating absolutely everything in the world that might be causing symptoms." It's the very opposite of that, actually. It is a dietary experiment to see whether FODMAPs are a trigger for your gut symptoms. If it helps, great. Work with it. The goal is not the most virtuously limited diet (no caffeine, no gluten, no red meat, no fat, no FODMAPs, no, no, no...). The goal is the most nutritious, delicious and varied diet you can tolerate.
FODMAPs are one potential trigger for digestive distress. Gluten is another. Some people might learn they need to limit both in their diets, others not.
If you've been gluten-free for a while before starting a FODMAP elimination diet, its easy enough to continue until the right time comes to "challenge" with gluten to find out whether it truly is a symptom trigger for you.
If you have not been on a gluten-free diet before starting FODMAPs, there is no need to try to be 100% gluten-free now. "Cross contamination" and traces of wheat in your food won't hurt your FODMAP experiment a bit. In fact, if you stay gluten-free for more than a few weeks during the elimination phase of the low-FODMAP diet, you might interfere with future blood tests for celiac disease if your doctor decides they are indicated.