There is no mechanism of gas production in the stomach.
In intestine, gas can be produced because of bacteria fermenting carbohydrates but in stomach nothing like that happens.
Only possible source of air or gas in stomach is air that is ingested from mouth during eating or chewing.
If one eats with open mouth, 3-5 ml of air is ingested with each swallow and just imagine how much air can be ingested if one is not giving attention to it.
Other possible source of air or gas in the stomach is carbonated beverages like Pepsi coca cola etc. These must be avoided.
If eating style modification still is unhelpful, then other possibility remains gas produced by bacteria inside intestinal lumen and refined carbohydrates like white bread, cake cookies, pasta, pizza, rice etc should be avoided and more complex carbohydrates like whole wheat bread, fiber etc should be preferred.
Yogurt with acidophillis also helps .
Metamucil, more liquids and daily rexercise is helpful in most of the cases along with simethicone( mylanta).