In the G650ER at max TO weight (103,600 lbs) initial cruise alt is FL410/FL430 depending on direction of flight. Seeing we don't always need 7,500nm range, with typical 5-7 hour fuel load we normally cruise FL450/FL470... For better rides we find ourselves at FL490 from time to time. Not really an issue for the plane. FL510 is elusive though, you have to be light and ISA has to be good.
The plane flys great at FL470 at .88 and a cabin alt of 4,200ft ish.
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