Why? if the B747-8i can be flown with a B744 rating, why can't Gulfstream put some effort into getting common type ratings?
From what I understand, the G650 is fly by wire whereas the 550 is conventional. The FAA decided that was a big enough difference to warrant a new type rating. Gulfstream would have been delighted with a common rating as it would have been a sales point. The fact that the holder of an existing 550 rating can do a truncated course is in no small part down to lobbying by the good folks from Savannah.
The 747-8i/F is simply a rehash of the 747 400 with a slightly different wing and different engines. The flight deck instruments have also been updated.
I have just done the 747-8 differences course. The biggest difference is the electronic flight bag and electronic checklist and that's not a big deal.
Same situation as the 737 300/400/500 and the 737NG which can be flown on the same rating. The differences in the flight deck displays, as well as the performance of the NG were much more marked than the 747 which is just a bit heavier and with more poke in the engines. The other major differences are a RAT and some clever automatic stuff with the hydraulics not requiring any pilot input, and which makes no difference to flying the thing.
A FBW system is a lot more complex and requires a significant change to the checklists and QRH. Hence, new TR.
The Falcon 900 and 7x look much the same but have a different type rating for the same reason.