Dante, a powerful demon, uses his powers to manipulate Gluttony, forcing him to eat his own heart. This leads to Gluttony's demise.
It's important to note that the anime adaptation of Fullmetal Alchemist: Brotherhood does not follow the manga completely in this instance. In the manga, Dante uses a different strategy to defeat Gluttony.
The anime focuses on a more dramatic and visually impactful scene, highlighting Dante's power and the tragic fate of Gluttony.