GTX is a an envelope gate effect but with extra options that allow you to use it with multiple Buses / FX-routers.
If you are using for more than one output be sure to select the right total of devices and also set a different device# for each instance.
You can also split by group by using the groups selection. If you want to manually select which device will trigger set the Threshold to MAX (full right).
The Envelope uses the Attack, Hold and Decay values.
The Threshold is used to set the level on which any sound above triggers the envelope.