GWD is very buggy on Windows 10. However, in the last few days I have been able to get some faces to my watch. It seems to have a lag time that I need to account for.
Also, make sure the Author and Distributor Certificates are signed in.
You will need to enter your Author AND Distributor certificates EVERY time you open Gear Watch Designer in order to successfully install watch faces.
I personally avoid using the "run on device" button in GWD. It likes to freeze and sometimes doesn't even bother to work.
This face installation guide was a great help.
Gear Watch Designer Face Installation Guide | Samsung Gear S2
This youtube video helped also.
https://www.youtube.com/watch?v=QoYykc6vqzQ