it´s not necessary to use two midi channels!
you can use only 1 channel and map the patches for the g-major in its midi-menu. for example: you hit the 3rd button on your midi-contoller which does normaly sent PC#3 on channel one. if you want the g-major preset number 53 called up with the 3rd button, enter the midi menu, turn the right wheel until you can read "Prg Bank" (use the custom bank mode) press the right button, after that "Prg Chg 1" shows up, (turn it to "3" if it´s not allready displayed), turn the wheel again and now you can read "Map to ....." turn the right button again until "53" appears. store and exit.
now the g-major patch "53" is called up if you hit the 3rd button on your midi-controller.
for the amp gizmo: just press the desiered options (channel, boost, reverb) and press store again.
your patch is now ready to rock!
you´ll not get any problems by using only one midi channel.
i used my complex rig with only one midi channel and it works well.
it´s easier and quicker if you run it on one channel, and the amp gizmo can handle up to 128 presets, this should be enough.