“What are the first signs of gluten intolerance?”

This is controversial topic. Gluten "intolerance" is not well defined medically. The biggest source of confusion is that when gluten in wheat is ingested, other carbohydrates like fructans are also ingested and the latter may cause symptoms more than the gluten, but it's impossible for the patient to know which is the culprit. Trying to determine if someone has a food "intolerance" is a frustrating process because the science is not worked out and you will get many medical and non-medical opinions that are not based on evidence. My personal approach to a patient who comes to me with a complaint of gluten intolerance is to first make sure that they don't have celiac disease, then reduce dietary carbohydrates, which are often the source of symptoms more than gluten specifically. See this important study that illustrates the above points: https://pubmed.ncbi.nlm.nih.gov/29102613/