In Curse Of Strahd, the Pack heard of Gertruda after talking to Mad Mary and adopting Lancelot, but never made it to the castle to rescue her. As with many of Strahd's interests, she stayed in his room until he eventually bored of her. Not interested in marrying the naive girl, he sequestered her as a Vampire Spawn in the catacombs; Gertruda was trapped in her own eternal tomb below the castle until the Slaying Of Strahd.
The lone daughter of Marya Pavlenka, Gertruda grew up sheltered by her mother, and was never allowed to leave Mad Mary's Townhouse as a child. Screened off from the world, the girl was nearly delusional in her naivete, with a very black-and-white view on morality. She made snap judges on appearance and immediately trusted every person she met at their word. Taught by her mother to only focus on positives, she did not believe in many evils of her world, including the existence of Vampires.
Her curiosity festered after seeing the Count Strahd von Zarovich in the Village of Barovia, taunting the villagers and terrorizing the Barovia Burgomaster's Mansion. Gertruda was raised to believe that evil could never triumph over good, but was charmed by the vampire lord. Thinking him a prince, she slipped away from the Townhouse and made her way to Castle Ravenloft, drawn by its majesty. When she did so, she left behind her only friend Lancelot, who wandered after her but was trapped in the Death House, forever separated from his master.