Long ago there was an exceptionally beautiful Vesya (prostitute). Her beauty made the rich and the famous run after her. She had a strict principle. She will consort with only one man in a day and will treat him like her husband for the day and will never entertain a second man on that day no matter how much money was offered. She was a great devotee of Lord Vishnu.
One day Lord Vishnu wanted to test her. So he went to her in the guise of a rich man and offered a generous sum. She invited Him in to her house, got him seated and prepared a scented warm water bath and fresh clothes to wear. As he took of his clothes for bathing it was seen that he had leprosy. Seeing this her mother shouted at him and was about to drive him out of the house. Gandaki stopped her and gave him a bath and then gave him food to eat. Soon he got high fever and died. As she accepts every man whom she agrees as her husband for the day, she prepared his cremation and got ready to enter the funeral fine to immolate herself with him as was the tradition of those days. As she entered the fire with the dead body the fire turned in to flowers and Lord Vishnu appeared in front of her. Overjoyed she fell on his feet and offered prayers. A pleased Lord Vishnu asked her to ask any boon she wanted. She asked that Lord Vishnu should be born as a so in her womb. Lord Vishnu granted her the boon and said that in her next life HE will be born in her womb.
In her next life Gandaki was born as a holy river and Lord Vishnu took birth in it as the holy Saligrama.