My new 1250 (january this year) has just started to develop the squak on front pads. That kind of noise is not new to me and I had it on my last 2 bikes and have it on my car right now.
As I worked as car mechanic many years ago and kept on developing my skills as amateur mechanic on motorcycles, I can only say what I know about that squeak. The squeak is produced by tiny vibration of the pad. Any pads will vibrate but the vibration is so tiny that human eye or touch can't see/feel it. That vibration produces high frequency metal-to-metal banging that at the end is revealed as a squeak. But, the banging is not between the pad and the disk. I mean, it is happening there but no noise is produced. The noise is produced between rear of the pad and the piston where the pad is touching. At that place, it is metal-to-metal. So that is the source of the squeak.
The solution is very simple, it just requires some work and patience. You just have to remove the pads and put tiny film of copper grease at the area where the pad is touching the piston. You can purchase copper grease in any automotive store. When I do it on my bikes. I put the grease at the area I mentioned and at any other area where moving parts are touching metal-to-metal on the calipers. So far, I had success of 100%. The noise has never come back.
If we lived in perfect world, the factory would have the copper grease applied on new bike and at the service where pads are replaced, they would apply it. But they don't. You can see on some bikes that there is tiny plate installed on the rear of the pad with some cushing material to decrease the vibration. I don't know if GS uses it as this is my first BMW bike and I haven't looked at the pads yet.
I like to say that I know a lot about mechanics on cars and bikes but few months ago I just made rookie mistake. My car, Ford, went to its 15.000 miles service to the dealership. That was the last service I've done there because the car was still on warranty. Next service, the car is not on warranty anymore and I'll do it myself. So, they called me and said that pads need to be replaced. My mind was completely ocupied with the work that day and I just said OK, do it. What a mistake. Firstly, pads didn't need replacing. And secondly, my car has horrible squeak now. I'll have to do copper grease now on my car and on my bike...
One more piece of advice. Do not sand the pads surface or disks because you'll need to do bedding process on your pads again. The best way is to put them back the way they were and on exact position they were before removing. You can just wash them with soapy water if you wish. Sanding may resolve the noise for few miles but after bedding process is over, the squeak will return.
I'm not professional mechanic. What I said here is just my humble opinion...
