I can understand your frustration especially since you have undergone so many tests. If your gastric issues are heartburn/constipation/gas/bloating etc, then it could be IBS. The diagnosis is made by ruling out Crohn’s and ulcerative colitis through endoscopy and colonoscopy. Possibility of H pylori infection should be looked into through a carbon urea breath test—though endoscopy often detects inflammation.
Food intolerance can be to wheat, oats, corn, protein, egg, milk and milk products, citrus fruits, tomatoes, grapes and many other such food items. Diagnosis is through a food tolerance test. Another way to do so is to maintain a food register/diary. Log in the food you eat and the symptoms as they appear. See if there is a correlation between your symptoms and a particular food type. Please discuss with your doctor.
I sincerely hope you will find this information useful in your journey towards better health.
Hope you get well soon! Good Luck and take care!