I wonder how many of those LOIs are people staking a claim in anticipation that the waiting list will be ridiculously long (given the current waiting time for the G550 and competitors) and how many of them are hoping to recycle their position closer to delivery to a more impatient customer.
If the $500k is as stated fully refundable, then it might be a decent punt (interest foregone versus chance of selling an early position?). Though I find it hard to believe that anyone right now is paying a $500k deposit for an aircraft ten years down the production line. Serious customer or gambler.
I'd also have thought there isn't a cat in Hell's chance of the completion centres turning out anything like that number of aircraft of that size in any sensible timescale. Whatever Gulfstream can do to speed up the green deliveries, that must be the pacing item, surely.