You can adjust how rapidly it drops off by adjusting the Attack/Decay setting. For a fast drop off, reduce the number (minimum is 10 milliseconds = 1/100th of a second). For a more gradual decay, increase the Attack/Decay (maximum 1000 milliseconds = 1 second).
You need to set the “Threshold” lower. Note that the threshold is in -dB (minus dB), so a more negative number is a lower threshold.
The “Level Reduction” control will adjust how much to reduce the level of the “silence”. If you want the silence to be really silent, adjust it all the way to -96 dB. If you only want a small reduction in volume, move it to the right.
To avoid them being “wacked off”, the Threshold should be set as low as possible but still catching all the low level noise, then the Attack/Decay setting can be adjusted to create a “fade out” rather than an abrupt stop.
There is an updated version of the Noise Gate available here: Noise Gate - #3 by steve
It also contains a tool to help determine a suitable level for the Threshold setting.