The 250 has GENERAL, fixed, gb. Garrett has decided, to make its machine "plug and play", so they set the gb for a general value. GB is the trade name for a feature that matches the machine circuitry to the conductivity of the ground under the coil. By doing this, the machine can better "see" the change that occurs when a metal object, under the coil, changes the return signal that was sent out. The change is usually exspressed as a change in sound. This is good for specialized machines, such as "gold detectors". Bad for detectorists that just want results but don't want to be bothered with the "whys". TTC