Tuna,
Historically Gulfstream has consistently been known to under promise and over deliver, so I wouldn't be surprised if we do see some of these range estimates being surpassed, but most likely I suspect it will be the high speed range numbers that could be improved, as was the case with the G650 that was given a revised M.90 range of 6,000nm up from the initial 5,000nm once the aircraft had been in service a short while.
In the case of the 650 the overall max range was never surpassed, but it has been found that it can come very close to this max range at M.86/M.87 instead of the published M.85.