Make a comparison between the G500 and the G450, for the same fuel the G500 will take you 800 miles further at a higher speed and a bigger cabin, that's because of the new designed engines and wings...
Doesn't G450 vs G500 prove my point? G500 has the bigger cabin and faster, carries a couple percent less fuel and yet manages 15% better fuel efficiency. The G550 is a newer plane with a much better wing than the G450, but the G600 should still be able to perform better relative to it. Instead of 2% worse efficiency, like I said I would expect a few percent better.