The amended lawsuit, filed by the late 22-year-old's conservator Bedelia Hargrove, alleges Gordon, 25, of "being out all night on a cocaine and drinking binge" and giving Kristina a "toxic cocktail," reported Us magazine.
"I am deeply saddened by the recent death of Bobbi Kristina Brown. I have filed today an amended complaint against Nicholas Gordon adding a count of wrongful death and for pain and suffering.
"We will leave no stone unturned in seeking justice for Bobbi Kristina Brown," Hargrove said in a statement.
The only daughter of the late Whitney Houston and Bobby Brown, Bobbi Kristina died on July 26, nearly six months after she was found unresponsive in a bathtub of her home.
She was buried next to her mother in New Jersey.