Barb, the 24" scale length of the GT automatically means less tension than other models at 25.5" or even 24.8" for the GC. The reduction is about 9-10% overall. Mediums at 25.5" scale are about 180 pounds total, and at 24" scale they become about 162 pounds -- almost exactly the number for 53-12 light gauge on a longer scale length. That is why the GS Mini (23.5" scale length) comes with mediums as standard. It takes a heavier string to offset the shorter length. As Dennis said, tuning down one half-step across the board accomplishes the same thing. You are back to the tension of light gauge. Tuning down a whole step is even looser.
It is doubtful that you would have to change the setup unless the thicker strings are binding in the nut (pinging as you tune up or down). Worst case, you might need a quarter turn on the truss rod, but probably not. Most of my Taylors wore light and medium gauge, with no ill effects.
