The GPU passthrough is relevant because it means that you don't need to build out a whole machine for just windows to isolate it. Instead you put a GPU in either your server (or your main machine) pass it through to a windows VM and then stream from it (through something like steam big picture, nvdia streaming, sunlight/moonlight, etc) knowing that the only info Microsoft slurps up is gaming related. You can even isolate windows from the internet if your gaming habits don't require it.
The game output is pushed out of the GPU exactly as if you were playing on bare metal. Streaming assumes some form of processing and pushing bits down a pipe.