Gideon's father is Mr Prewett and his mother is Ms Anne Prewett. His paternal grandparents are <Unknown> and <Unknown>. He has a brother and a sister, named Fabian and Molly.
Gideon was a member of the original Order of the Phoenix, but, according to Alastor Moody he died as a hero with his brother. Both are in a picture of the original Order members that Moody shows to Harry. They were brutally killed by five Death Eaters, including Antonin Dolohov, who went to Azkaban for it.