The singer is now the proud owner of a 2008 ivory silk gown titled "The Girl Who Lived in the Tree," according to Britain's Press Association.
Guinness says, "I'm overwhelmed by how many bidders took part ... and I'm particularly moved by Lady Gaga's support for the foundation. I like her very much, and it's lovely to see the nascent beginnings of something that may help other people."
The auction raised $747,200 for the Isabella Blow Foundation, set up in memory of the fashion guru.
Keep clicking to see some of Gaga's notorious ensembles ...