use feature : GPUsMonitor broken on Windows
The GPUsMonitor metaknob doesn't work on Windows because the path is set to $(LIBEXEC)/condor_gpu_utilization instead of $(LIBEXEC)\condor_gpu_utilization.exe. Instead, conditionally set GPU_MONITORcorrectly and use that.
CODE_REVIEW looks good. I like that you moved the path to the gpu monitor into a knob.