This may be a bit late but you CAN view sketchfab models on Gear VR through the browser... though it's not great... not very accurate and doesn't follow your head motions well. Anyway, here is how you do it:
Download "Package Disabler Pro" (I think it's a dollar or two) from the google play store and check the little check box next to "Gear VR Service." This will disable the Gear VR app from automatically starting when you plug the device in.
In sketchfab (in your browser), set it to VR mode and just put it into the Gear VR headset and your all set!
YOU DO NOT HAVE TO ROOT YOUR DEVICE IF YOU USE THIS APP