How to use G-API with GPU?

edit

there is a sample here, which shows changing the backend to "fluid", and to use the opencl backend, you'd do similar:

add a few headers:

//! [opencl_includes]
#include "opencv2/gapi/ocl/core.hpp"            // ocl Core kernel library
#include "opencv2/gapi/ocl/imgproc.hpp"         // ocl ImgProc kernel library
//! [opencl_includes]
#include "opencv2/gapi/ocl/goclkernel.hpp"      // ocl user kernel API

change the backend and run the graph:

// Prepare the kernel package and run the graph
cv::gapi::GKernelPackage ocl_kernels = cv::gapi::combine        // Define a custom kernel package:
    (cv::gapi::core::ocl::kernels(),                            // ...with ocl Core kernels
     cv::gapi::imgproc::ocl::kernels(),                         // ...and ocl ImgProc kernels
     cv::unite_policy::KEEP);

//! [kernel_pkg]
//! [kernel_hotfix]
// probably not nessecary for ocl !
//ocl_kernels.remove<cv::gapi::imgproc::GBoxFilter>();          // Remove ocl Box filter as unsuitable,
                                                                // G-API will fall-back to OpenCV there.
//! [kernel_hotfix]
//! [kernel_pkg_use]
segm.apply(cv::gin(imgIn),                                      // Input data vector
           cv::gout(imgOut, imgOutCoherency, imgOutOrientation), // Output data vector
           cv::compile_args(ocl_kernels));                      // Kernel package to use