Hi,
Since hardware engines in Jetson series do not support BGR, so you would need to use software converter to convert to the format. Please check discussion in
[Gstreamer] nvvidconv, BGR as INPUT
For better solution in using OpenCV, please execute sudo nvpmodel -m 0 and sudo jetson_clocks. The commands can enable CPU cores at maximum clocks so that software converter runs at max throughput.
For using jetson_multimedia_api + OpenCV, you may refer to this patch:
NVBuffer (FD) to opencv Mat - #6 by DaneLLL