Let's try the following:
# Turn on both the Wii U console and the GamePad and wait for the console to get to the home screen.
# On the GamePad, tap "Controller Settings" on the left.
# Tap "Settings" under Wii U GamePad.
# Adjust the screen brightness by tapping the "+" and "-" icons on the screen.
# When you are finished adjusting the brightness, tap "Back" on the Wii U GamePad, then tap "Close HOME Menu".