Tuna I don't think you are taking proper account of the aircraft's increased speed and increased size
G600 @ M85 = 6200nm (38,760lbs max fuel ~ 6.25 lbs per nm)
G550 @ M87 = 6000nm (41,300lbs max fuel ~ 6.88 lbs per nm)
That to me makes the G600 approximately 10% more fuel efficient than the G550, while at the same time offering the passenger 10%+ increase in cabin volume.