Figure: GV Automatic Emergency Descent, from GV Aircraft Operating Manual, §06-04-00, figure 1.
I've had one rapid depressurization over the years — at FL 350 in a Boeing 707 — and believe all the practice in the simulator you can get comes in handy. There are a few things most of us pilots don't understand about the requirements, the priorities, and the process. A better understanding could save your life.
Notice a recurring theme? Don't believe those times of useful consciousness charts, you are not going to last that long. Read Abnormal Procedures / Rapid Depressurization to find out why.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
[G450 Airplane Flight Manual, §04-21-20]
NOTE: An initial pitch attitude of 8° to 10° nose down is recommended to commence descent. As speed approaches MMO / VMO, extend the speed brakes to maximize the rate of descent. Adjust pitch attitude as necessary to avoid an overspeed condition.
Consider leaving the autopilot engaged and using the touch control steering (TCS) button to override its inputs to turn and point the airplane downward while the PM adjusts the altitude select to the desired lower altitude.
NOTE: It is recommended that the emergency descent be flown manually in order to achieve maximum descent rate.
You are better off leaving the autopilot engaged — if you pass out the autopilot can get you down to altitude, level off, and push the power levers forward. Even if the speed brakes are deployed, the aircraft will maintain speed until you recover.
[G450 Airplane Flight Manual, §04-21-30] The autopilot has an automatic Emergency Descent Mode (EDM) that is armed anytime aircraft altitude is greater than 40,000 feet with autopilot selected ON. When the red Cabin Pressure Low warning message is displayed on the Crew Alerting System (CAS) and aircraft altitude is greater than 40,000 feet with autopilot selected ON, the following occurs:
As the nose heads downward try to consider the aircraft's structural integrity and evaluate if a VMO descent with the speed brakes deployed will do more harm than good.
NOTE: The pilot may override EDM by disconnecting the autopilot.
You should leave the autopilot engaged and use the touch control steering (TCS) to get the airplane pointed where you wanted — if you pass out the autopilot can get you down to altitude, level off, and push the power levers forward. Even if the speed brakes are deployed, the aircraft will maintain speed until you recover.
Figure: G450 Oxygen duration chart, from G450 Airplane Flight Manual, §01-35-00, figure 1.
How low do you go? Well, how much oxygen do you have? The G450 oxygen chart in the AFM Limitations section only provides crew duration, on the theory that you will get down to 15,000' where the passengers can - legally - go off oxygen:
Figure: Oxygen durations, from G450 Operating Manual Supplement G450-OMS-02, Table III.
But if you want to know the right answer, you will have to look at the Operating Manual Supplement OMS 2, Extended Range Operations, where you will find this gem:
14 CFR 121, Title 14: Aeronautics and Space, Operating Requirements: Domestic, Flag, and Supplemental Operations, Federal Aviation Administration, Department of Transportation
14 CFR 135, Title 14: Aeronautics and Space, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft, Federal Aviation Administration, Department of Transportation
FSI G450 PTM, FlightSafety International Gulfstream G450 Pilot Training Manual, Volume 2, Aircraft Systems, October 2008
FSI G450 MTM, FlightSafety International Gulfstream G450 Maintenance Training Manual, August 2008
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Gulfstream G450 Operating Manual Supplement, G-450-OMS-02, Extended Operations (ETOPS) Guide, Revision 2, April 2, 2009
Gulfstream G450 Wiring Diagram Manual, Revision 17, October 31, 2012