How can I use DNN module of opencv at GPU on Jetson Xavier NX


this is return:
[ WARN:0] global /home/nx-1/opencv/modules/videoio/src/cap_gstreamer.cpp (935) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-1
[ WARN:0] global /home/nx-1/opencv/modules/dnn/src/dnn.cpp (1363) setUpNet DNN module was not built with CUDA backend; switching to CPU

For GPU support you need to build OpenCV manually. There are several threads about this here in the forums and also some scripts for the build process available, e.g. this one.