Gastric and intestinal gas exchange
Intestinal gas consists of ingested air and gas formed during digestion. Stomach gas consists of  15% oxygen and 7% carbon dioxide approximately; the remaining is nitrogen.
The air that we breathe contains about 21% oxygen; thus some of the swallowed oxygen is absorbed through the blood capillaries present in stomach. Carbon dioxide is synthesized by digesting food using gastric juices. Nitrogen is not absorbed.
The small intestine absorbs some amount of carbon dioxide and oxygen. And the remaining gas is passed further to the large intestine. In the large intestine,  most of the oxygen is removed and carbon dioxide is increased. New gases are produced mainly hydrogen because of bacterial fermentation. And these gases are absorbed by blood and later released via lungs.
Check out following links:
https://katesalevelbiology.wordpress.com/2017/02/14/human-gas-exchange-the-digestive-system-and-breathing/
https://www.gastrojournal.org/article/S0016-5085(19)33654-6/pdf
https://journals.physiology.org/doi/abs/10.1152/ajplegacy.1926.76.1.92
https://nyaspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1749-6632.1968.tb19024.x