I’d recommend using a different port (e.g. 5610) just to make sure there aren’t any conflict issues with companion’s default streaming functionality or QGC’s receiving.
Better to try things one at a time/in isolation. First try receiving with gstreamer from the commandline (e.g. use the gst-launch-1.0 program and output to autovideosink) to confirm whether your pipeline is correct and that the stream is reaching your computer, and separately check the obs-gstreamer is working properly with a test pipeline (e.g. videotestsrc ! video).
When I was working with OBS-gstreamer previously, I found that it uses a different version of gstreamer to QGC (MinGW instead of MSVC), which can cause some compatibility issues if you’re building both. I’m not sure if you’re also building QGC, but given it’s giving errors about a MinGW gstreamer it’s potentially a related issue.