Hi,
Please check if you can run this python sample and see camerapreview:
OpenCV Video Capture with GStreamer doesn't work on ROS-melodic - #3 by DaneLLL
If yes, you can adapt your gstreamer pipeline to the sample and try again. For comparison, you can also try to run the gstreamer pipeline in gst-launch-1.0 command and replace appsink with fakesink. To check if the pipeline works fine in gst-launch-1.0.