itk:GPUImage::Graft cannot access protected member declared in class itk::GPUImage

I am starting my registration project. I modify the code: Examples/RegistrationITKv4/DeformableRegistration16, and it work.

Then, I want to use GPU to accelarate the program, and replace the DemonsRegistrationFilter with GPUDemonsRegistrationFilter, then the bug is reported:

error C2248 ‘itk::GPUImage<VectorPixelType,3>::Graft’: cannot access protected member declared in class ‘itk::GPUImage<VectorPixelType,3>’ d:\itk_gpu\insighttoolkit-5.0.1\modules\registration\gpupdedeformable\include\itkGPUPDEDeformableRegistrationFilter.hxx

Could somebody provide a demo for GPU based Demons registration?