All Landsat 7 ETM+ bands, (excluding thermal band 6) have an adjustable gain setting, which maximizes the instrument's radiometric resolution for each band without saturating the detectors. Generally, acquisition of a scene in low gain allows a higher dynamic range with less risk of saturation over bright areas. Acquisition in high gain will provide greater radiometric resolution (sensitivity).
Landsat 7 thermal band 6 is acquired in both gain states and the image data is provided in two separate band files (6L and 6H).
Gain setting files can be found on the Landsat 7 Long Term Acquisition Plan web page.
More information about gain change can be found in the Landsat 7 Science Data Users Handbook.