While smart:gate's special feature is its content-aware processing, the plug-in can also be used as a conventional gate. If you select the "No Target" profile (this profile is pre-selected by default for every new instance), smart:gate works source-independently and reacts to the entire input signal.
So, to use smart:gate as a conventional, level-driven gate, select the “No Target” profile and set the Level Bias to its maximum value (100) to make sure that the the Activity Function (which serves as the control signal for the gating) actually follows the level of the input signal.