Gestahl is an ambitious and ruthless man who desires nothing less than total control over the world. Gestahl personally led an invasion of the esper world twenty years before the start of Final Fantasy VI and he ordered his men to abduct espers with intent to experiment on them and drain their powers. When Gestahl saw the collapsed Madeline thrown out of the esper world, he took the infant Terra from her arms and laughed as he realized she was a half-esper child, and that he could use her to accelerate his plans. When Madeline stood up and called out to stop him, Gestahl coldly told her to be quiet and struck her (it is ambiguous if he killed her or merely knocked her down).

Gestahl is a charismatic speaker and a consummate liar. He inspired genuine loyalty in hundreds if not thousands of Imperial soldiers, and convinced the Returners he truly wanted peace and wanted to negotiate a truce with the espers; though they did not fully trust him at his word, they went along with his plan. In the Cave to the Sealed Gate, Kefka Palazzo claimed that Gestahl purposefully let Terra escape the Empire and join the Returners, as part of a plan to get them to open the Sealed Gate for them. It is unknown if Kefka was telling the truth, but the suspicious evacuation of the Imperial Observation Post and Kefka's timely arrival at the Sealed Gate right as the Returners get there would suggest that if Gestahl had not intended this all along, he was at least able to anticipate their actions and set a trap for them.

Under Gestahl's rule the Empire has invaded many cities and killed thousands, and he shows no regret or remorse over these deeds. While others in the Empire loathe Kefka for his cruelty and bloodthirsty nature, Gestahl trusts him for the same reasons and has no issue with Kefka's actions, which often include the pointless deaths of the Empire's own troops. Gestahl cares little for the Empire except as a means to acquire power for himself. While the Imperial capital Vector burned from an esper attack, Gestahl remained in the Imperial Palace and threw a banquet. Given the opportunity to seize the Warring Triad, Gestahl abandoned Vector and went to the Floating Continent to claim them, and had the Imperial Air Force defend the continent from his enemies. When he felt that Kefka, the last ally he had, could no longer be trusted, Gestahl attempted to kill him.

Gestahl's objective is to rule the world, which causes him and Kefka to turn on each other when Kefka tries to awakening the Warring Triad, as that would devastate the world and Gestahl has no interest in that. Gestahl's final words before Kefka kicks him off the Floating Continent differ between the Super NES and Game Boy Advance versions. In the Super NES script, he says "there'll be no one to worship us", a final attempt to reason with Kefka and convince him not to go through with his actions. In the Game Boy Advance script, he says "the world is about to learn the meaning of fear", showing terror over what Kefka is about to do and the consequences humankind will suffer for it.

Gestahl collects paintings and commissioned an artist in Albrook to do his portrait. However, the Emperor was dead by the time the artist completed the work, so it was sold to Owzer in Jidoor and put on display in his mansion.