A few interesting points have been made here, however,
The Global 7000/8000 are scheduled for EIS in 2017/2018, and the G650 will start deliverers in a few months. So you just might take delivery at the same time if you order either one today. (The 7x doesn't compare to either in size or range).
The backlog is around 36 months for a G450 or G550. So buyers wanting a Gulfstream will have to wait regardless. Gulfstream may also entertain a purchase that includes a lease of an aircraft in the inventory (i.e. used) until a month or two beyond delivery of the new aircraft, so you can get flying today.
Regarding the deposits, owners must continually increase their deposit as the aircraft nears production.
Finally, I'm not sure of sales prices, but I was lead to believe that the 7000/8000 would be more expensive than the G650.