I know several of the Gulfstream test pilots, and am deeply saddened by this news. I'm dreading the release of the crew manifest.
Would there be additional Flight Data Recording going on during this phase of testing?
Just might give a clue as to what in the world happened.
They would have been recording a huge amount of flight test data, but the recorders almost certainly don't have any specific fire protection. So it may take a lot of work to retrieve any of the flight test data. Depending on what type of test they were doing, they might have been transmitting telemetry, which would normally include quite a bit of data, but not as much as would be recorded on the flight test recorders.