Except it's not historical, the south didn't actually fly that flag at Gettysburg.
Actually, it was the battle flag of the Army of Northern Virginia for the entire war following First Manassas, and since the Gettysburg campaign was an ANV campaign it most certainly was flown there.
I thought the NVA flag was the square jack? The rectangle jack was a navy flag as far as I know.
I'm pretty certain at least one southern regiment flew this flag at Gettysburg.