We just completed an 8 hour round trip to take my daughter and dorm friend and cello back to school.
The only way we could get the cello to fit was to stand the hard case on its side and put the neck through the small fold-down panel over the rear armrest. Forms a bit of a wall in the back seat, but stable and quiet. The cello is too long to go in trunk and point forward, and won't fit sideways or diagonal in the back.
Thank you, BMW - in our Prius this would have been a major packing inconvenience with a very cozy ride for the two rear seat passengers. As it was, everyone was very comfortable. Except for the driver in heavy holiday traffic, but that's a different story.
Other ideas for configuring large items in a GT? (no, the instrument is not riding on a roof rack...)