Gluten allergies are relatively rare and extreme. People with Celiac have a major, sometimes life threatening reaction to gluten.

People who do not have Celiac and say they are gluten sensitive usually are not. They are either experiencing a nocebo effect or have gas from other components of wheat rather than gluten. Actual non Celiac reactions to gluten are minor if they exist at all. So when someone says they are gluten sensitive it is overwhelmingly likely they either have Celiac and even a very very tiny amount of gluten will cause them horrible consequences, or they are incorrect and they are not sensitive to gluten at all.

https://pubmed.ncbi.nlm.nih.gov/27523634/

https://pubmed.ncbi.nlm.nih.gov/29102613/

More on the "other components" issue -- a lot of people have issues with some of the fermentable carbohydrates (e.g. fructans) in wheat, which also happen to be present in the other gluten-containing grains. "Have gas" is a bit of an understatement. For some people this can lead to cramps, vomiting, and other fairly severe digestive problems.

So people on the low-FODMAPs diet end up using "gluten free" as a convenient shorthand for "doesn't contain these particular grains".

20 years ago I was "that guy" who went "uhm, actually, you almost certainly don't have celiac". Alas, my efforts did not stop the trend, and today the nocebo effect for gluten has become a multi-billion dollar racket.

To be fair, the people with psychogenic gluten intolerance have massively increased the availability and variety of gluten-free foods for people with real Celiac disease. They seem to be doing more good than harm on balance.