5er GT sales are around 25,000 units worldwide. In some markets the demand is high, in some there is almost none. It depends. But it sure isn't a mainstream car.
The demand for 5er GT in Europe is OK, but the offer still lacks lower-end diesel engine to be even more affordable & attractive for customers - eg. both CLS and A7 (which are in the same price segment, although not the same concepts) come with such diesel engine in European markets (both with 204 HP), while entry diesel-powered 5er GT is 530d with 245HP, followed by 535d with 300HP. Btw, A7 & CLS sales are between 30,000 and 35,000 units annually - with quite high sales in US market, where 5er GT is a complete fail.
There has been a 525d prototype spied a while ago, so I guess 5er GT will also come as 525d GT with 218HP, and accordingly a bit lower price compared to 530d. This will sure boost sales. Especially among targeted customer pool - the elderly who appreciate roomy & comfy BMW with semi-commanding sitting position.
But 3er GT will sure be much more successful seller in Europe anyway - it's closer to Europeans: car size, engine choices etc. 5er GT is HUGE.