You can use the Wunderlich multi-pod adapter. This replaces the bolts that hold the ends of your handle bars in place.
MultiPod Adaptor (#1250269) K1600 GT 1250269 | BMW Wunderlich America
Once you have this installed you screw in a rammount ball for attachment. I don't specifically remember which rammount ball you need, perhaps someone else can chime in.
Or you can use the Wunderlich MultiPod Modular, which is a nice piece of engineered equipment but also really expensive that you would use instead of the rammount.
MultiPod Modular (#1250267-268) C600 Sport 1250267-268 | BMW Wunderlich America
There's also an Adaptive Mount that will let you mount various things on it. For instance I've got on of those on the right side with their Universal Grip Holder, that's been holding my iPhone.
Use model D-01-10 for the GTL
AdaptivMount Horizontal Control Mount<br>(D-01-10)
Techmount also makes a mount.
TECHMOUNT - Products
Good luck!