From the rumour mill and therefore "alledgedly":
I can't remember what they said the mechanic was doing, but he was cleaning something wearing an oxygen mask. When he took it off whatever he was doing didn't get on well with the O2 coming out of the mask and it (alledgedly - Gulfstream have more money for lawyers than I do) caused a flash fire which engulfed the rear section of a (I believe) Luton-based G550.
The back of the aircraft was superficially damaged (smoke etc), but the problem was that the hell hole door was open - so they don't know where the flames got to inside.
Closed everything down for at least 24 hours - if spares shipment was anything to go by.
Will be interesting to see whether the G550 owner wants it back or not after that...