I've seen three good places to mount a Stebel on the GT.
The easiest is under the left body panel, on the shelf in front of the ABS pump. There's lots of room there on the '07 and newer bikes. The '06 bikes had a larger power ABS pump, but there is still room behind or above the ABS pump.
The next place is under the right body panel, just below the ZFE module. It's a bit tight, but it does fit in there. Plus, that's near the original horn, where the new relay trigger wire ties in to the stock wiring.
The third place is up in the nose cowl, under the windshield adjustment mechanism. Again, it's a tight fit, but does work.
I also know a couple of guys that split the horn's motor/blower assy from the plastic horn section and used a rubber hose to connect the two. That gives more flexibility in terms of mounting locations, but can add a delay depending on the length of the extra hose.
Mine is under the right body panel, since my ABS shelf has four HID ballasts taking up that space. I sometimes wonder if it's not quite as loud as it could be, since it's pressed up pretty close to the actual fairing panel (you need room for the air to move to make the sound). I've thought about moving it to the nose cowl, but haven't bothered yet.
There's lots of info if you search for Stebel here or over at the K12/13GT forum
at K-bikes.com. Specifically, look at post #11
in this thread
for pics and ideas.