It's sort of like a volume control or a sensitivity control. On the G90 there are 2 modes, head lock and standard rate. If you are not using the radio remote gain feature, the pot on the gyro sets the gain and the mode. if you are using the remote gain feature the pot on the gyro is disabled.
Do you need instructions on the use of the gain, or are you asking for theoretical reasons? I mean, "how does this work" can be answered many different ways, depending on whether you want to build a new one, sell one to somebody, or use one....