I have seen a lot of comments about the G-DECs being noisy and thought I might write down what I have noticed myself and some input from other users that I have read in different forums.

Regardless of what type of pickups you have you will notice a lot of hum or buzzing when you plug in your guitar. Actually if you just plug in your cable without having the guitar plugged in you will get this noise. But, this is not true for all the presets.

This noise might have different sources: computers, TVs and other home electronics; from your mains; nearby radio/TV antennas etc. Some of this noise can be reduced by moving your amp to another power outlet or just yourself a few feet... But this story is more about the settings on the G-DEC and how they affect noise output. 

Typically when you switch on your amp the preset P00/U00 "Rocking G-DEC" will be active and this is one of the noisy settings. The reason is a combination of high gain and compression, and in some cases also high guitar pickup volume. 

First of all, and easiest to do is roll off you guitar pup volume a little, for me with CS Strat 56' relic, it helps a lot. Next step is to tweak the settings on the G-DEC.

If you examine the "Rockin G-DEC" preset you will se that it uses Amp type=British 3, Gain=9.5 and Compressor=Low. The noise gate is also set to low. Amp types are numbered 1-3 and higher number equals higher gain (True for all but the DYNATOUCH2 setting). So selecting an amp type with lower number will result in lower gain, as well as it changes some of the other parameters. Easiest way to examine this is to use the Preset Editor and change amp type while studying what happens to the different settings.

So, first select amp type, and then set the other parameters.

The Compressor function is to limit the dynamics of the signal. That means that it reduces the amplitude of signals higher than "average" and amplifies signals lower than "average". So the compressor will amplify lower amplitude sounds, as noise, more than it will amplify the wanted signal from your pups. So there will be a tradeoff between sustain and noise... 

Then, at last, apply the level of Noise Gate you desire.

Also important to remember is that, a preset that sounds good for one user might not sound the same for someone else, because it is so dependant on guitar and pickup combinations.