Why are my renders being rendered by my CPU and not my GPU? In this screencap you can see I have GPU Compute selected but my rendering is being done by my CPU.  Does the fact that GPU Compute is grayed out mean GPU acceleration isn't available on my card (GeForce GTX1080, supposedly supported by Blender), or are there other configuration issues?  Thanks for any tips.
You are enabling Open Shading Language.
Currently it's only support by CPU instruction.
Disable it first.

Please check your preference setting,

Check only GTX 1080 for CUDA device, save the preference and you might need to restart your Blender (most of the time don't need).