Noise Gates - Attack, hold and release

The most advanced settings on the gate relate to attack time, hold and release time. This affect the shape of the gates behaviour.
Let us take our example of a Tom Tom. When you strike it, the initial transient is quite high and then the Tom fades away for a time depending on the way it is tuned. This can sound very muddy through a PA system so the gate is often used to try and make the sound much more 'clicky' than it does by default. If however you simply set the threshold for the drum, you might find that the sound is made very short and transient and loses all its resonance since the sound quickly drops back below the threshold. By adjusting the hold time, you are telling the gate to stay open for a short time after the key drops below the threshold (usually micro or milliseconds) and then close. This gives you the basic functionality to allow a little bit of 'ring' from the drum before slamming the gate but then you might have a noticeable slamming of the gate which makes the drum cut off very abruptly and not very musically. What you can do to resolve that is to add a slight release time which slows the shutting of the gate.
The attack time is used less often and is like the opposite of release, it sets the time taken for the gate to fully open and would be used if the level of the sound is sometimes very close to the threshold of the gate and causes noticeable sudden gate opening and stuttering. By slowing the attack time of the gate, you allow more subtle variation but this would not generally be used on transient sounds like drums where you need the gate open as soon as possible to avoid cutting off the transient.