Lots of people feel the Tutorial in the manual is too complicated. Here's a 
process starting with an existing preset to create a custom patch. This tutorial 
will show you how Amp and Cab selection is done for the green and red channels
in a preset, how Gain, Tone and Level are set for Red and Green channels, how 
to tweak FX on/off settings and how to store the preset when you;re done.

All my tweaking and subjective choice making is based on listening to an ES-335 
into the GNX, into the computer via S/PDIF and out of the computer on my Event 
TR-8's at moderate volume. I'm sure your mileage will vary :) but hopefully the 
info here will show you what you need to start crafting your own signature tone.

So- lets start with an existing preset- Factory Preset #32, BLUEBALL. You 
probably know that there are a couple ways to get to that Preset: Regardless of 
what mode the GNX is in (Stompbox, Bank or Recorder) the data wheel moves you 
through the presets (unless you're in the process of editing a preset or 
recorder config). When the GNX is in Stompbox mode (Yellow LED's on the Up/Down 
switches) the Up/Down footswitches move you through the presets too.

When the Green User LED next to the datawheel is lit, you're looking at User 
presets- you can edit/save/whatever these. By default they start the same as 
the factory presets. When the Red Factory LED is lit, it's a Factory preset. 
You can mess with the parameters but you can't save your changes into the 
factory preset. You can however start there, and save into a user preset, which 
is what we'll do.

So, put the GNX in Stompbox mode (chord footswitch 4&5 until Mode and Up/Down 
LED's are yellow, or press the Mode switch until LED's are yellow). Then 
datawheel to Factory 32.

In this preset, the Red Channel uses the HIGAIN amp and Vintage 4x12 cab; Green 
Channel is BLUES amp and BLND 2x12 cab. You see this by pushing the 
"Red/Green/Yellow" button in the FX matrix 3 times and watching the display-  
After the third press the display will show, in sequence, Green Amp, Green Cab, 
Warp, Red Amp, Red Cab. In this mode, tweaking the 'Green Amp' knob changes the 
Green Amp model, and so on.

By default when you get to this preset, the Red amp channel is selected, which 
you can tell by the Red LED being lit on the 'Amp' footswitch. EQ, Noise Gate, 
Reverb and Delay are on , which you can tell by looking at the LEDs for the 
individual FX in the matrix. You can also tell that Reverb and Delay are on by 
noticing the LEDs on the 'Control' and 'Delay' Footswitches.

Before we do anything, play a few riffs with your axe set on the bridge pickup, 
tone all the way up, volume at 3-4, then with the volume at 9 or 10. Then hit 
the Amp footswitch to go to the Green channel and do it again. For me, the Red 
channel is pretty crunchy at just about any Guitar volume setting where the 
Green is much more dynamic, with a lot more scream coming on based on guitar 
volume setting and pick attack, I think this is a quality we'd like to exploit 
for this preset.

You'll also notice (I think) that the Delay is really inappropriate for what we 
want. Hit the Delay footswitch to turn it off.

I'm not wild about the HIGAIN amp model for blues use, and in most cases I like 
to set up my red and green channels to use the same Amp and Cab, but vary their 
settings so that it's more like a real red/green channel- set the gain higher 
on the red, so that you can stand out for your solo... So let's change the red 
channel's amp and cab settings first, to be the same as the Green:

 Making sure that the Yellow LED is lit on the "Green/Red/Yellow" button  in 
the FX matrix, twist the rightmost knob (Red Cabinet) until you see BLND 2x12; 
Twist the next knob until you see BLUES. Twist the middle knob so that WARP is 
1. You've just changed the Red Amp and Cab.

 Now the Red Channel sounds pretty weak. What I'd want to do next is to look at 
the Gain, Tone and Level settings on the two channels. I want to make the Red 
Gain significantly higher, make the tone settings pretty much the same on both 
channels(maybe a little more top and mid on the red channel) and then (maybe) 
adjust the Red Level so that the extra Gain doesn't blow your ears out.

 Push the Red/Green/Yellow button once. The knobs along the top of the matrix 
are now adjusting Green Channel parameters (we know this because the LEDs are 
green). The display is showing the values for the various settings: BLUES 12 is 
the Amp model; Green Gain is 70, Tone is Flat all the way across, Level is 79.

 Push Red/Green/Yellow again and we see that Red Gain is only 50 and Level is 
67. Let's take those Gain and Level settings up. Twist the leftmost knob in the 
matrix until you see the numbers changing, and set the Gain to 90; Twist the 
Rightmost knob and set Level to 79. You can (and should!) listen to the effect 
this has on your tone as you adjust the Gain and Level, making sure you try 
different guitar volume settings and pick techniques.

 Push the Red/Green/Yellow button again, and we're back to "Normal" stompbox 
mode.

 I also do not like the Noise Gate for my blues playing, and if we're going to 
turn that off, we should check the EQ settings while we're in that part of the 
FX matrix. Using the Effect Select Buttons at the left of the FX matrix, 
repeatedly push the buttons until the Equalizer LED is lit- the Display will be 
scrolling through the EQ settings- you can see what the band settings are for 
the EQ, and what the levels for the bands are (all zero, no harm there). Push 
the down Effect Select button again to get to Noise Gate. The display shows you 
that it is "On". Pressing the Red/Green/Yellow button now turns it off.

You can now either repeatedly push the up/down Effect Select buttons until 
you're "out" of the matrix, or push the Exit button to the right of the data 
wheel.

 At this point, I think the preset is sounding pretty good. Both channels can 
go from clean-ish to scream-ish based on guitar and playing, and it's a pretty 
straightforward, uncomplicated blues tone. Saving the preset is a multi-step 
process- You hit Store, you name your preset, you choose a user preset location 
for it, and then you actually store it. So:

 1: Hit 'Store' (to the lower right of the data wheel)
 2: Name Your Preset: Notice the display shows "BLUEBALL" with the B Flashing. 
Spinning the datawheeel changes the chosen letter; pressing "Mode" or "Drums" 
(their LEDs should be flashing) moves you forward and backward through the 
letters of the Preset's name. When it says what you want, hit Store.
 3: Choose a User Location: The numbers in the display should be flashing 32, 
and probably the Name (if you changed it) has gone back to BLUEBALL, this just 
means that User Preset 32 already had a "BLUEBALL" model in it. At this point, 
spinning the datawheel moves you through the user prest locations, showing you 
the name of the preset already stored there. Once you find a user preset you 
don't mind overwriting, press Store again.

 Voila, custom user preset created. I hope this helps, let me know how it goes! 

 PS. Since I wrote this I decided that the preset is TOO hot, and lowered both
Red and Green gain.