Hello,
I have a digitech RP-2000 and have occasionally used it though amps over the years. Your processor is essentially a pre-amp and when used through a guitar amplifier the best thing you can do to get a "pure" tone is to go from your GNX (left out) to the effects return of your amp. This should bypass the preamp section of your guitar amp. If it's not possible to do this then plug the GNX into the clean channel of your amp. If your amps' clean channel also has a gain setting; keep the gain low and use the master volume to increase loudness. Hope this helps.
sincerely,
Doug