Hi,
We would suggest use dsexample plugin. Please check
/opt/nvidia/deepstream/deepstream-6.0/sources/gst-plugins/gst-dsexample/README
And an refer to the patches of calling NvBufSurface APIs:
How to create opencv gpumat from nvstream? - #18 by DaneLLL
Jetson Nano CSI Raspberry Pi Camera V2 upside down video when run an example with deepstream-app - #7 by DaneLLL