The Sixth District seat, once held by Newt Gingrich, was at the center of a special election fight in 2017 that became the most expensive House race in history. Representative Karen Handel, a Republican, won that election, and her opponent declined to try again. So Ms. Handel faces Lucy McBath, a former flight attendant who was motivated to run for office after her son was killed in an act of gun violence nearly six years ago.