Be very careful with the 3 channel switching, I looked into this recently and called mesa. I'm not sure if the G major switches channels by using a midi configuration but if it doesn't then you may have a problem. As msi stated the 3 channel rectos default to channel 2, but you can't simply plug into channel 1 and 3 ftsw jacks and start tapping away because you can't engage both of those channels at the same time. Doing so turns both channels on at the same time which is very bad for your amp. Mesa explained to me that those jacks were designed for using with a midi setup so that you could hit a single switch which would (via midi) turn off the other channels while engaging the channel that you want.
edit****
Ok so I downloaded the manual for the G major, and in fact it looks like you can control those channels via midi, so I think the only thing left to figure out is if you can come from the single stereo out from the G Major to two mono connections and have channels 1 and 3 respond to the msgs. Which is part of what I think what lailer75 was referring to. If you can, with the right midi ftsw controller I think you would have an awesome setup. Stomp on a button and control what effects and their respective settings along with what channel you're amp is on. Pretty Sweet!!!