How far can you go on a tank of gas? We all know that depends on a number of things but having a no-wind, Mach 0.80, ISA, optimal altitude number in your head will get you started.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
Figure: Twin Engine Flight Planning for Mach = 0.80 at ISA, from G450 Performance Handbook, pg. PB-18, with Eddie's crayons.
Note that the chart does not reflect ASC 016 which raises maximum ramp weight to 75,000 lbs and maximum takeoff weight to 74,600 lbs, so you can expect to do slightly better.
You can fly 4,000 nautical miles at M 0.80 on an ISA day with no winds and have 5,000 lbs of fuel on landing. Of course you seldom have those conditions. The charts are not friendly for this kind of work, but going between the charts in the AOM, AFM, and PH you can make a few general inferences:
Of course you cannot assume your conditions will remain constant and your best tool may be your flight planning service. My longest flight in the G450 was a 10.0 but that was with ideal weather and lots of airports in the Boston area to choose from. I rarely plan anything requring more than 8.0 and I'll do 9.0 if conditions are in my favor.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Aircraft Service Change 016A, Maximum Takeoff Gross Weight Increase, May 30, 2012
Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011