Yeah, stay away from the GT8 if you have a good amp. God knows, they sound like shit for effects. What the hell does Thump know about tone, anyways?
Seriously... I have both, the G-major is for sale. $300 and it's yours. It's a little more hi-fi, but the GT8 has more effects, is a hell of a lot more versatile, and even without the amp modelling I prefer the flange and chorus on it. Maybe I'm just used to Boss stompboxes, but the GT8 works better for me.
Pete