it depends on whether you're in single or live mode.
I haven't touched my G in a while, so i kinda forgot all the technical terms, but i can tell you the steps i did when programming suvch a patch myself (ages ago).
In single mode, make sure the patch you're using is made up out of only 2 elements / tones. You're going to need the other 2 for the feed-back effect. On the 3rd tone slot (i'm not sure if it's tone, ot element...i've been switching a lot between keyboards lately), add a simple sine wave, with the volume set at 0, and pitch +7. On the 4th tone slot add another sine wave, with volume set also at 0, and pitch +12, and also detune it by a few cents.
Then, the rest happens in the modulation matrix - just add whatever controller (aftertouch, of mod wheel) you want, to alter the volume setting, setting a maximum value of around 100 for the +7 sine, and a maximum value of around 60 for the +12 sine. You also need to set the same controller to decrease the volume to tones 1 and / or 2, so that when you increase the "feed-back", the original tone is dimmed a bit.
In Live mode it's a bit easier (you still have to create a patch made out of two sine waves as decribed above), but then just load the patch you want in a Live mode channel, and the sine patch you created in another channel.
Then, off to the modulation matrix again (IIRC it should be somewhere in "part view - controllers", but correct me if i'm wrong). In there make sure to add the same amount i described earlier to set the volumes for the sine wave patch, and the original patch. The only thing to keep in mind when creating the patch for live mode is that you will be controlling the volume for both patch tones from the patch level, so you need to set the tone volumes in the sine patch to -6 for the first sine, and -12 for the second.
I hope i've not confused anyone with my lingo. I've been working on a lot of diferent synths lately, and while the basics are similar to say the least, the technical terms are diferent, and remembering them all is harder than just remembering guidelines to follw :).
