ImageMagick/Magick.NET will use OpenCL when this supported by your video card. Support for this is detected automatically. You can disable this with MagickNET.UseOpenCL.
But there is no such thing as "Use only GPU". Magick.NET will only use GPU acceleration for a subset of the operations. You can see for which operations we do that here: http://trac.imagemagick.org/browser/ImageMagick/trunk/MagickCore/accelerate.h.
But there is no such thing as "Use only GPU". Magick.NET will only use GPU acceleration for a subset of the operations. You can see for which operations we do that here: http://trac.imagemagick.org/browser/ImageMagick/trunk/MagickCore/accelerate.h.