Matthew Waters on the GStreamer forum has answered this question. I quote him below. Ah, so your problem is use with the Qt designer not showing finding the relevant qml code. That use case is currently not supported by qmlglsink. The issue is that the necessary QML item is contained within the qmlglsink element itself and is registered dynamically when the element itself is instantiated. Qt designer doesn't know anything about GStreamer so cannot perform this initialisation for you. There are no qmldir or qmltype files for qmlglsink. The 'fix' involves are rather complicated restructure of the qmlglsink element to support placing the qml part/plugin in a separate .so that is findable with Qt tools (using qmldir or whatever).