Hi,
By default there is OpenCV 4.1.1 installed and it enables WITH_GSTREAMER=ON. This sample works by default:
Doesn't work nvv4l2decoder for decoding RTSP in gstreamer + opencv - #3 by DaneLLL
Looks like yo re-build OpenCV. Please make sure you enables gstreamer. May try this script from a user:
GitHub - mdegans/nano_build_opencv: Build OpenCV on Nvidia Jetson Nano