It's either in CM or HA, iirc.
She was never cursed, and was born as a Divine Spirit, except that unlike her sisters, she wasn't immortal and unaging. She was exiled to the island by Athena due to jealousy or some such, and her sisters came along. She became the Gorgon due to killing and consuming the blood of those who came to the island to take her sisters away, or, banking on the infamy she had received from Athena, try to kill her. She ended up consuming her sisters too and just existed as a monster until Perseus came and killed her.