All Landsat 7 ETM+ bands, (excluding band 6) have an adjustable gain setting, which allows the Mission Operations Center (MOC) to maximize 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).
Band 6 (thermal infrared) is acquired in both gain states and the image data is provided in two separate band files (6L and 6H). (More details at the FAQ describing this)
Gain setting files can be found on the Landsat 7 Long Term Acquisition Plan page.
Gain state details can be found at the Landsat 7 Science Data Users Handbook page.