The threshold control is calibrated in decibels. 0 dB refers to the gate’s nominal input level. This means if your gate has +4 dBm I/O, then if you set the threshold to 0 dB, the gate will open when the box input exceeds +4 dBm.
If the threshold is set to -15 dB, then the gate opens when the input is above -11 dBm. Likewise, if your box has -10 dB I/O, then the 0 dB mark means -10 dB.
Next is the attack time. The attack time control sets the time (usually in microseconds, or milliseconds) the gate takes to go from closed (maximum attenuation, as set by the range control) to open (zero attenuation).
The gate doesn’t “wait” the attack time before snapping open; rather, it smoothly ramps the attenuation from max to zero in the attack time. This is analogous to starting with the channel fader on minimum, and fading up to unity, in microseconds.
If you’re playing along at home, you may notice that if the threshold is set so that it’s barely below the drum input level, the gate will “click” as it opens. You can mitigate that click with the attack time control.
Here’s what happens: when the signal goes above threshold, the gate is told to open. If the attack time is too fast, the gate output wants to switch instantly between 0V and some non-zero value—maybe a couple of volts. Now, if you use the attack time to slow down the attack, that voltage doesn’t change “instantly,” but rather smoothly ramps up.
So, the trick is to set the attack time fast enough to capture the drum’s transient, but not so fast that the gate clicks.
The hold time control is obvious—it’s simply how long (in milliseconds) the gate remains open once it’s fully open. Too short a hold time, and you clip the end of the drum’s ring. Too long, and the gate may not close, or you’ll get excessive ring, or what have you.
You can also get weird clacking noises from the gate as it chatters. Too short a hold time and you’ll find the gate might try to open and close and open and close quickly.
The decay time is the same idea as the attack time, except it determines how quickly the attenuation increases once the signal goes back below threshold. Too quick a decay time and you’ll clip off the drum sound tails. Too slow and it may not be closed before the next drum hit.
The range control is what sets how “closed” the gate is. When set to max attenuation (say, -80 dB; negative dB gain is attenuation!), when the gate is closed, there is 80 dB attenuation from the input to the output. That’s closed!