What is the G scale?

The G scale indicates the strength of the geomagnetic storm. I goes from 0 to 5. (0 being the weakest, 5 being an extreme geomagnetic storm).

G0 = Kp 4 and below = None
G1 = Kp 5 = Minor
G2 = Kp 6 = Moderate
G3 = Kp 7 = Strong
G4 = Kp 8 = Severe
G5 = Kp 9 = Extreme

Visit NOAA for more information about the G-scale.