As far as I understand.. A noise gate cuts the output when you go below a certain input which is defined by the threshold level that you would set with your potentiometer. When you're playing, the noise or hum is still part of your signal, you just can't hear it as bad because you're hearing the guitar on top of it. So it will still be there. When you stop playing, your signal gets cut out completely, so you don't hear any hum.
The noise gate creates an envelope governed by the attack, hold and release volume/time parameters. Attack is the amount of time it takes to "open the gate" once you supply an input higher than the threshold level. Hold time determines how long the gate is open for and release determines how long it takes for the gate to close once the input falls below the threshold.
All of those parameters have an effect on how your signal will ultimately sound. Obviously it will affect your dynamics. If your hum is quite loud, you will have to turn up the threshold level a lot, and thus you decrease your playing dynamic envelope. ie, you can't play softly, if you do, your signal doesn't come through. Having experienced it in guitar rig, its not something that I feel that I have any time for..
Noise suppression is something quite different I think, and is probably more what you're looking for. Its method is concerned with attenuating offending frequencies from your signal. There are tons of different ways of doing it.
This looks quite decent:
http://www.ehx.com/products/hum-debugger
What I would do though is try to first reduce the hum on your guitar and amp.
edit: someone got there first.
