Gluten is a group of proteins found in grains like wheat, rye, spelt, and barley.
There are two primary reasons that gluten could be causing you issues:
1. The form you're getting it in - white bread is a primary example. The flour is highly processed and hasn't undergone the necessary fermentation making it harder to digest (hence why the only bread I recommend is sourdough, which has undergone this fermentation).
2. The amount of it you eat - here's where is get's really interesting... with enough exposure to certain foods, the protein contained in them can become "invaders", and thus a sensitivity can be developed - which is why variation and rotation of food sources is so important.
Given the rise in food sources containing gluten, especially wheat given how cheap and easy it is to produce on mass, we must be mindful of our total consumption - and not just due to sensitivities...
Exposure to enough gluten for long enough also leads to issues such as leaky gut, which is where the lining of your gut is compromised and things seep through that aren't meant to be there (bacterial endotoxin) which results in inflammation and an immune response.
This is because gluten triggers a protein called zonulin, and this makes the holes in the lining of your gut bigger (to keep it simple) - which is what leads to leaky gut (increased intestinal permeability). Something I had when I had IBS (irritable bowel syndrome).
Research also finds that those who suffer from IBS see significant improvement in symptoms on a gluten-free diet - hence why gluten is one of the sources we initially eliminate on my coaching plan The Gut Reset.
However, research also finds that it may not be the gluten that you're sensitive to, but FODMAPs (fructan [the F in FODMAP] specifically in the research) - which are a group of carbohydrates that are commonly poorly digested. Again, a food group we eliminate and re-introduce on The Gut Reset.
Celiac disease is an autoimmune disease that can be pictured as a severe form of a gluten intolerance - where your body sees gluten as an invader which the immune system attacks. In this case it's black and white and can be tested for.
It also may not be gluten that you're sensitive to, but wheat! Which is a form of gluten. If someone who is sensitive to wheat went on a gluten-free diet they would feel better, and hence they might build a false association with gluten as a whole - when wheat was the issue.
So, I have three recommendations for you: 1) Limit the consumption of processed foods like white bread, pastries, (some) pasta (fresh is better), etc. and opt. for fresh, fermented, and less highly processed options. 2) Rotate sources of gluten in your diet if you are going to consume them. 3) Keep all things stable in your diet and try removing/introducing gluten in different forms to see how you react, and act accordingly.