Is it possible to develop for Samsung Gear VR in C++ only and possibly combine it with Qt? According to http://www.gearvrf.org/ there's a Gear VR native library, but is it supported in any way (samples, documentation..) ?
These are actually two questions:
Is it possible to develop for Samsung Gear VR in C++ only?
Yes, definitely, but I would prefer Oculus native mobile SDK over your mentioned framework. Because at current stage, it is really Oculus who are pushing the software development forward, you know John Carmack, right? And he also brings a lot engineering friends from ID software to Oculus too, which you will come across a lot when reading the code distributed with the SDK.
https://developer.oculus.com/documentation/mobilesdk/latest/concepts/book-native/
Combine with QT?
I don't think there is any benefit now, if what you need is to build menus, buttons, UI elements like this. UI elements are very different in VR world, which the graphic is all about a 3D based world. And I think if you are following the native development path, you can only count on some preliminary UI support from Oculus too. Again, you can only get help by reading the source code in the mobile SDK from Oculus.