The read-only Local GPU field displays the graphics card installed locally, if there is one available. You can toggle the GPU on and off using the Use GPU if available control. If you have multiple GPUs available, select the requried device using the Preferences > GPU Device dropdown.
NOTE: Selecting a different GPU requires you to restart Nuke before the change takes effect.
TIP: Even if there is no GPU available locally, you can still enable Use GPU if available. BlinkScript then uses the GPU as and when one becomes available.
You should also enable this if you wish to render from the command line with the --gpu option.
Additionally, enable Vectorize on CPU to use SIMD acceleration on the CPU where possible. See Help > Documentation for more information on kernel execution.