Not able to read Video from Capture Card with GPU (Unsupported Format) while Reading with CPU works fine

C++
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I could not say for sure without testing myself, there may be other arguments which need to be set, have you tried stepping through CvCapture_FFMPEG::open() to see which FFmpeg call fails and what the error code is?
Is your webcam /dev/video0? You should be able to test it with

ffmpeg -f v4l2 -i /dev/video0

or similar.