Bushy, there is a site devoted to the G-Dec here: www.mygdec.com/, user registration is free. If you go to the section titled "file repository" (on the left), and then to the G-Dec section, there is a .pdf tutorial of sorts available for download called G-Dec Going Deep which might help you out. You need to be registered to be able to download.
What I did with mine, is I picked a preset to goof with that I wasn't planning on ever using, and then spent an hour or so finding out what the buttons did, how to change the effects and settings, what it all sounded like, etc. If you don't think of it as homework, it becomes kind of fun.
For old-time twangy country, you might try the preset called "trainbilly" (that's what it's called on mine), which is a country/rockabilly sort of sound with a bit of reverb. You might also try adding a bit of reverb to the "acoustic" setting as well, and maybe a bit of delay (100ms or so). You're definitely not going to want to mess with phasers or chorus effects, though as I said, it's kind of fun to just play around with them.
It really is a fun little amp.
When you think the time has come to move on from it, you should keep two things in mind for a twangy country sound: all tube, and an echo pedal (I personally like the Roland Space Echo. It ain't cheap, but it sure adds flavor and doesn't sound "digital" to my ear. YMMV).
"There's such a fine line between genius and stupidity."
--David St. Hubbins
