As a coeliac who was diagnosed with negative bloods. And a positive endoscopy.
The whole gut digestion thing is a minefield.
Overall - what aided my diagnosis was a comprehensive food diary. I’d recommend this to anyone with gut troubles. Also - don’t take no for an answer if you’re still having issues, push for further tests or second opinions.
As for gluten, some people decide not to include it in their diets and believe they have a sensitivity. Plenty of literature around this.
@jcw You could try a food allergy test that might help.
I found out a year ago I was allergic to gluten but I also have to avoid egg, milk, tomato, although according to my doctor once my gut has healed enough I should be able to eat them again, except gluten which is a life long exclusion. One thing I have found is that going completely gluten free is not easy as they put wheat in a lot of things you just wouldn’t expect.
I have to agree with @Pott55 and don’t take no for an answer, I ended up going to a private (functional medicine) doctor because my GP didn’t help me, I believe there are many people with food intolerances that cause them problems and they may not realise it’s what they are eating.
FWIW, I’ve had issues when going 100%, going 100% for a few days leads to toilet issues and bad gas as you say, 90% huel, so 3 huel shakes and an evening meal, absolutely fine. I think my stomach just needs something solid at least once a day.