When the Great War ended in the Mahabharata, Gandhari was the
most distressed. All her sons were killed, her brother was killed and
emerged victorious, she lost many near and dear ones Bhishma and other family
members. Death of Gandhari happened soon after. She along with Dhritarashtra
and Kunti took Vanaprastha – they went to the forest to perform intense
austerities.
Gandhari died in forest fire. This is mentioned in the Ashramavasika
Parva of Mahabharata Chapter 36 – 37.