@DeepakVishak
SGaist is right. custom GStreamer pipelines have been dropped in Qt 6 Multimedia, however you can achieve the same using qml6glsink.
Have a look at this example:
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/tree/1.24/subprojects/gst-plugins-good/tests/examples/qt6/qmlsink?ref_type=heads