GMax is a maximizing limiter. It is designed to boost the level of the signal without letting the peaks exceed a given maximum.
The Gain parameter sets the desired boost in volume. The signal will be boosted by this amount before going into the limiter.
The limiter reduces the signal level as necessary to keep it below that specified by the Ceiling parameter. The Release parameter sets the time taken for the limiter to recover after turning the volume down.
A meter is displayed on the GUI showing the gain reduction applied by the limiter. The resolution of this meter is 1dB per bar.