How do I make my GPU, and not CPU render eevee and material preview?
Your computer will use your GPU for material preview and Eevee- exclusively. There’s no way to enable CPU rendering for material preview or Eevee, so conversely, there’s no way to not use GPU. If you’re not seeing GPU usage with Eevee rendering, make sure you have your latest GPU drivers, and be aware that Blender doesn’t report GPU usage accurately 
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