It's a cold soak flight being conducted by Gulfstream test pilots. The cold soak is one of the final flights in the Completion Test Flight series and is done just before the delivery of the jet to it's operator.
Generally, sound and temperature surveys are done on these flights, as well as SatCom registration and making sure everything in the cabin and cockpit works.
The initial Completion Test Flights are very similar to a Production Test Flight (Green Card Flight). Production Test Flights insure that the aircraft meets the requirements of Part 25 for airworthiness and must be successfully completed before a Certificate of Airworthiness can be issued.
If a customer requests it, Gulfstream has a test card for Customer Acceptance Flights.
GV