What do I do if my baby has gas after feeding?

Thats Normal. Baby will trap a little air every time it swallows. Thats the way the swallowing mechanism works. As baby gulps it down, you either burp it up or will deal with is exiting the other end. The digestive tract is just a long tube. Some parents swear by mylecon( gas drops) but its not magic. It may combine some bubbles but it doesn't stop the process.
Besides burping... Mylicon sometimes helps, sometimes does nothing. If gas persists and causing fussiness, baby should be checked for a milk protein allergy or see if there is a constipation issue.