Indeed, Gandhari never removed her blindfold. However, there is one instance from where one can estimate the power she from her penance. It was not for rendering Duryodhana's body hard as thunderbolt, but charring Yudhishtra's toe. This took place after the war when Gandhari was lamenting the loss of her sons.
From Jalapradanika-parva in Stri-parva
Conversant with the rules of righteousness, the Kuru queen, possessed of great foresight, directed her eyes, from within the folds of the cloth that covered them, to the tip of Yudhishthira’s toe, as the prince, with body bent forwards, was about to fall down at her feet. At this, the king, whose nails had before this been all very beautiful, came to have a sore nail on his toe. Beholding this, Arjuna moved away to the rear of Vasudeva. and the other sons of Pandu became restless and moved from one spot to another.
It's to be noted that the very next section (Stri-vilapa parva)mentions that Gandhari was then bestowed with power of vision from Vyasa.
Gandhari, though staying on that spot which was distant from the field of battle, beheld, with her spiritual eye, the slaughter of the Kurus. Devoted to her lord, that highly blessed lady had always practised high vows. Undergoing the severest penances, she was always truthful in her speech. In consequence of the gift of the boon by the great rishi Vyasa of sanctified deeds, she became possessed of spiritual knowledge and power. Piteous were the lamentations in which that dame then indulged. Endued with great intelligence, the Kuru dame saw, from a distance, but as if from a near point, that field of battle, terrible to behold and full of wonderful sights, of those foremost of fighters.
As a result she could see the great maharathis and adhirathas laying on the field of battle. She was saddened on seeing their wives wailing over their bodies.