Despite the fact that his father was a slaveholder and that three of his brothers, two brothers-in-law and his cousin J. Johnston Pettigrew served in the Confederate military, Gibbon decided to uphold his oath to the Union.
Trods det, at hans far var slaveejer, og at tre af hans brødre, to svogre og hans fætter J. Johnston Pettigrew, gjorde tjeneste i den konfødererede hær, besluttede Gibbon at stå ved sin ed til Unionen.