Found it! From the [Adobe Premiere effects help]:
Dynamics effect The Dynamics effect provides a set of controls that
can be combined or used independently to adjust audio. Use either the
graphical controls in the Custom Setup view, or adjust values in the
Individual Parameters view. This effect is available for 5.1, stereo,
or mono clips.
Cuts off a signal when the level falls below the specified threshold.
Use this control to remove unwanted background signals in recordings,
such as a background signal in a voice-over. Set the gate to close
whenever the speaker stops, thus removing all other sounds. The LED
display colors indicate the gate’s mode: open (green), attack or
release (yellow), and closed (red). Use the following controls for
Specifies the level (between –60 and 0 dB) that the incoming signal
must exceed to open the gate. If the signal level falls below this
level, the gate closes, muting the incoming signal. Attack
Specifies the time the gate takes to open after the signal level
exceeds the threshold. Release
Sets the time (between 50 and 500 milliseconds) the gate takes to
close after the signal level has fallen below the threshold. Hold
Specifies the time (between 0.1 and 1000 milliseconds) the gate stays
open after the level has fallen below the threshold.