Now, as to why you would want to control the amount of attenuation: imagine putting a gate on, say, background vocals. When the person isn’t singing, the gate is closed and the background noise disappears.
When (s)he sings, the gate opens, and the background noise is present. It’s a bit odd sounding, so by setting the range to, say, -10, the difference between open and closed isn’t so startling.
It’s as if you pulled the channel fader down to -10. If you set the range to 0 dB, there is no gating action at all. Gates usually don’t have a “positive” range, since they do not add gain.
Now, for the frequency controls. These controls let you tune the frequency range the gate responds to. For example, say you’re gating the rack toms, and there’s a loud crash cymbal right above them.
With the frequency controls “wide open” (low at min, high at max), the gate may respond to the cymbals as well as the drum, which is not what you want.
So, you use the frequency controls to set the passband that the gate responds to. Thus for a rack tom, set the LF to something like 50 Hz, and the HF to something like 500 Hz (or whatever).
Since the majority of the cymbal’s energy is not in that passband, the gate won’t respond to it. But the rack tom does have a lot of energy there, and the gate opens when the tom is hit.
It is important to remember that the frequency controls do not affect the tonality of the audio! Also, some gates have slightly-different controls for setting the gate passband.
It’s similar to a parametric EQ – there is a center frequency knob and a Q control. The Q sets the width of the band centered on the center frequency. Some gates may have just the center-frequency control and a fixed Q.
Now, for some technical details: The gate is based on a device called a Voltage-Controlled Amplifier (or VCA). This is a device whose gain (and attenuation) are set by a Control Voltage. The audio signal through the VCA is usually called the audio path.
The Control Voltage is derived by something called the sidechain, which is parallel to the audio path. The gate’s input is split into two outputs; one drives the audio path and the VCA, the other drives the sidechain.
The sidechain uses a circuit called a detector to “look” at the audio and generate a DC voltage that corresponds to the level of the audio.
This voltage is modified by the threshold, attack, hold, decay and range controls to come up with the proper Control Voltage to set the VCA to do the right thing.