I've been asking this for over 4-years, and, while it would seem reasonable, the last time someone said anything, they said maybe after the LCI, which is happening now for the 2014 models.
One thing to keep in mind, the diesel engines sold here differ from those in Europe, primarily because of the sulphur in our fuel. While the standard changes some years ago here to lower it, it is still considerably higher than in Europe, so the engine needs different methods to clean up the exhaust. Yes, they do sell a diesel engine here, and yes, one would think it shouldn't be that hard to adapt it to the GT, but no, it hasn't happened. It may be that the market is so small for this vehicle, it's just not worth it to them, as opposed to a much higher volume sale with the 3 or 5 sedan, and in the X series. Amortizing it over such small volume, means it might end up costing more than it might change sales.
Maybe. I delayed purchase of my GT nearly two years waiting, then gave up.