I think Busterbucani has it right: 'high-speed' stall followed by not enough height to recover (the biggest airshow killer of them all).
Condolences to family and friends. the Gnat team are one of my favourite airshow acts, reminding me of the first time I saw a real jet (Red Arrows Gnat parked at the entrance to Brighton Pier in the mid-70s). I expect it was one of those detailed mock-ups like the later Hawk that used to do the rounds with the RAF Recruiting Team.
The only jets I had ever seen in real life were airliners like the Trident; that Gnat looked like a spaceship from the future as I was probably about 7 or 8 at the time!
Very grateful that the Gnat Display Team kept them flying
; I for one don't care how rich they are or what their individul motivations may be.
Flug