The biggest advantage of a buckler is that its strapped to your arm keeping a hand free while still gaining a shield bonus. You are also able to wear a gauntlet (normal, locked, or spiked) while keeping that hand free. Lets assume that the buckler is a +1 buckler so its providing a +2 shield bonus, and the gauntlet is just a +2.
The feat Shielded Gauntlet Master says
While using Shield Gauntlet Style, you no longer lose your shield bonus to AC when you attack with your gauntlet (or spiked gauntlet) or use it to hold a weapon. In addition, you add your gauntlet’s enhancement bonus to the shield bonus to AC granted by this feat as if it were a shield enhancement bonus.
So, does the bucklers shield bonus stack with the bonus from the feat? Since both would provide +2, is the final shield AC +2 or +4?