Insert the noise gate plugin of your choice onto the audio track that you would like to process.
Set the threshold control of the noise gate plugin so that it turns on once the audio of the track falls below a certain level. It is important to set the threshold correctly in order to get the desired effect out of the noise gate. If you set the threshold too high, the noise gate may attenuate the volume of the audio content that you do not want to be affected. On the other hand, if you set the threshold too low, the noise gate won't turn on soon enough to effectively attenuate the unwanted noise in the audio track.
Adjust the range or attenuation control to a level of your liking. The level of this control will dictate how much the volume of the audio track is attenuated when the noise gate turns on.
Adjust the attack and release controls of the noise gate plugin to dictate how quickly you want the noise gate to turn on and off once the audio signal has fallen below or risen above the threshold of the noise gate.
Adjust the hold control of the noise gate in order to create a smooth transition between the noise gate's on and off stages. The hold control dictates how long the gate will stay open once the threshold has been crossed (even if it immediately falls back below the threshold). This control is especially useful for keeping sustained notes from being abruptly cut off when the audio level falls below the threshold.