Residuals on most large BMWs have always been poor. The beautiful 6 series coupe and convertible that retail around £75,000 new well-specced are now under £45,000 after 12-18 months. The 5-Series GT is a niche model and does not perform in same bracket as the 520d saloon or wagon, so expect poorer residuals. I bought new and am absolutely delighted with the car and intend to keep it longer than 3 years. I think the GT is an excellent deal approved used especially if you buy in Yr 3 and keep for 3-5 years.
If we all bought cars with good residual values, we would be buying gray or silver 320d / 520d repmobiles in Eco trim and fat tyres and no spec. Fortunately, some of us are able to get beyond that