In Curse Of Strahd, the Pack found his body still hanging in the caverns, and cut him down after defeating Elizabeth. They escaped the Death House after killing Elizabeth and Walter; however, though he was destroyed in the resulting fire, the house's Curse remained.
The father of Rosavalda and Thornboldt, Gustav was the wealthy patriarch of the Death House. Having owned the Durst Windmill that fed Vallaki and the Village of Barovia, he was considered a pillar of his community. However, he hid a secret: he was utterly obsessed with gaining the favour of the Count Strahd von Zarovich, particularly after his ascendency as a Vampire.
For years, he co-ran the cult's activities along with his wife Elizabeth Durst. Growing prone to fits of insanity, he sired a bastard son Walter with his maid Zondra Zalenski, and drove his wife to madness. When he learned of what his wife did and what she had become, he committed suicide, hanging above his bed in the chambers below their house.