Hi,
Please check the discussion in
[Gstreamer] nvvidconv, BGR as INPUT - #4 by DaneLLL
The hardware converter does not support 24-bit BGR so you would need to convert to RGBA.
And then you can re-sample RGBA to BGR through CUDA code. We have sample code in jetson_multiemdia_api samples for re-sampling RGBA to individual B, G R planes. It is not exactly same as re-sampling RGBA to BGR, but you may take a look.