Aside from the GTL including items that you'll have to order as options when buying the GT, there are four main ergo differences.
Foot pegs: The pegs are set further back on the GT which appear about 2 or 3 inches further back than the GTL. Obviously intended to be more of a spot fit than the GTL.
Handle Bars I've never ridden an LT, but the bars are extended way back. Not sure how that works out, but perhaps some LT owners want to comment on the feel of those handle bars extending that far back. The GTL will have a similar bar set up as the LT. The GT bars however will not set back as far and may have a similar set up to the bars on the 1300GT or even an RT. So here again more forward and sport like for the GT.
Seats Can only tell by photos so far, but the GTL seat from the side view is deeper than the GT seat. The GTL seat is deeper and more U-shaped looking at the side view. The GT seat is straighter and more linear from front to back so again more of a sport set up.
Windshield Both bikes have completely different windshields. The GT windshield is considerably shorter than the GTL and narrower. .
Not sure if anyone knows how interchangeable the above parts are such as switching windshields or moving pegs forward or back. While the bikes have much overlap, the above differences dictate that you better have a clear idea of which style you want to own before you shell out a deposit. Based on the limited production, and high demand, most owners will be acquiring their K1600 by plunking down a deposit well in advance. It's just hard to discern how much of a sport feel you can get out of a GTL or how much long distance comfort you can get out of a GT. I'm sure for many the above differences pushes them further into their original choice of wanting either a luxury tourer or something more sport like.