GGate is a noise-gate effect. It silences the signal when its level drops below the threshold, given by the Thresh parameter.
You can specify the Fade, which is the amount of time taken to mute the signal, and the Attack, which is the time taken to unmute the signal.
When you are mixing several recorded tracks together, the little noise present in each adds up. During quieter passages, the background noise from "silent" tracks can become a problem. A noise-gate is a simple and convenient way to silence the parts of a track that are supposed to be silent.