In 1858, Gregg married Mary Francis Garth from Alabama, daughter of Jesse Winston Garth, a Unionist who was willing to give up his hundreds of slaves if it meant saving the Union.
Vuonna 1858 Gregg nai Mary Francis Garthin Alabamasta, unionisti Jesse Winston Garthin tyttären, jonka isä oli valmis luopumaan sadoista orjistaan pelastaakseen unionin.