The G force is the force of acceleration to which the device is subjected through motion
The unit of measure is the force of gravity, approximately equal to 9.81 m/s2 or meters per squared seconds.
Hence a reported value of 1G = 1 * 9.81 m/s2, or 0.5G = 0.5 * 9.81 m/s2, etc...
The measurements are taken from the device accelerometer, caused by motion only, excluding the force of gravity. At rest the value would be near zero
The application shows the instantaneous force, in units of G, in addition to the maximum attained forces while the application is running