I think this has been explained before but I'll go for it one more time. I was one of the first stateside to drive in a prototype and I have seen all the info there is so I am not just spewing....
The 5 GT is hitting its targets. The GT models do not specifically target Americans. The 5 GT model was designed and built for an aging population or those that desire certain attributes. They want a car and not an SAV, they like the easy entry and the better visibility the added ride height affords them as well as the adjustability- comfort to sport.
BMW has a physiology lab that has done extensive research into the best entry and exit angles; least load on the hips, back etc. The ride height and seat position of the GT models is the closest to this as possible. It is fascinating to see how they mock these things up and take all the photos.
The aging population in Germany in particular (largest intended market for such car) is wealthier and more active than past generations. Many studies have been done as to what this population seeks in a car and the GT models fill those needs.
The GT models are also beneficial for people to be driven in, for shuttles and for executive transport- they are comfortable, luxurious, spacious and easy to get into and out of (if you attend fashion events you see them). They also are more premium than a 5. There is also an increased number being used for car hire replacing Town Cars (stateside).
The 3 GT will be much more sporty and also target a new customer..... The Active Family. The A7 competitor is the 6GC and there will be a 4GC as well.