When a baby gets gas buildup in their stomach it's usually because the baby has sucked in too much air from the bottle when they are bottle feeding.
When a baby is bottle feeding they swallow air sometimes which is normal and that swallowed air then gets into the the digestive tract.
That's why you need to burp the baby after feeding them with a bottle because they could have trapped gas in their stomach that needs released.
When you burp the baby the gas gets released which helps the baby feel much better.
The baby can also swallow air other ways too which is normal and nothing to be really worried about.
When your baby has gas in their stomach then burping them usually releases any trapped gas.