Strong abdominal muscles are required to take a punch to the gut, as well as contraction right before the strike. There are many exercises suited to the task of developing core strength using only body weight. Weights can be added for further improvement in abdominal strength.
Magician Harry Houdini was famous for taking strong punches to the gut successfully. His primary technique consisted of bracing himself before the blow. Houdini had a student who challenged him to withstand a few punches, but the magician failed to brace himself for the blows and died from the injuries he sustained. Houdini's death highlights the danger of taking punches to the gut without being prepared.