The GPU Busy metric shows the percentage of time during which the GPU is busy. If this value is 100%, the analyzed workload is GPU-bound. A smaller value indicates a CPU bottleneck.
For GPU-bound workloads, the value of the GPU Busy metric is 100%. A value less than 100% indicates that the GPU is spending time in an idle state, waiting for data from the CPU (in which case your game or application could be CPU-bound).
If GPU Busy is consistently less than 100% and you are encountering performance issues, consider threading your game and using the Trace Analyzer to understand the interaction between the CPU and GPU.