System information (version)

OpenCV => 3.2
Operating System / Platform => Mac OS 10.12.5
Compiler => g++
Wrapper => Java/C++

It is impossible to instantiate a cvCreateVideoWriter_GStreamer because TRY_OPEN detects that a writer has already been created by ffmpeg (or possibly other cap). FFMpeg jsut creates a file with the name of my gstreamer pipeline construction..

My call to VideoWriter():
writer.open("appsrc ! autovideoconvert ! vtenc_h264 ! h264parse ! mpegtsmux ! rtpmp2tpay ! udpsink host=127.0.0.1 port=5000", fourcc, capture.get(Videoio.CAP_PROP_FPS), frameSize);

I can temporarily fix the issue by moing the HAVE_GSTREAMER check above ffmpeg, but I supose the real fix would be to have ffmpeg fail if it thinks that it should not be a valid file.