FYI There is a well known scenario where for a short time during startup before your setup code can run thatthe GIGA Display Shield will have a spurious display. If the board had been freshly powered up it may display random noise in the frame buffer. If that board was already running and you pressed the reset button or loaded new code then the last image from just before the reset will be displayed. These spurious images will disappear as soon as your setup code runs and displays something else.
More details and a fairly simple workaround are described here.
Giga Display Shows Garbage During Boot