Gain is the analog amplification of the voltage from the sensor before it is digitized. In a CMOS sensor a particular gain level can be applied to each line of pixels as they are read. The EXIF field is supposed to represent the "average" gain applied to all lines.
"low gain" means that the native optimal sensitivity of the sensor is being used and the dynamic range of the sensor is maximized. Up or Down is whether the average low gain setting is above or below the optimum.
"high gain" means that the sensitivity of the sensor is being boosted. For example, if the sensor has a native ISO of 200 and the amplifier is increasing this to anything above 200 (300, 400, 600, etc), then high gain is in use. When high gain is active, then the potential dynamic range of the sensor is reduced. Note that in a low light situation, using high gain may effectively increase the dynamic range of the captured image. As with the "low gain" setting, "up" or "down" refers to whether the average adjustment over all the lines is over or under the norm.