For OpenCV to use gstreamer it needs to know where it is. An alternative would be to copy the gstreamer dll’s into the same directory as your .pyd but I would not advise this.
Since python 3.8 dll’s can only be loaded from trusted locations
This means you cannot rely on using your system/user path and you need to explicitly add them with os.add_dll_directory. If your using conda or a similar distribution then it may automatically add the directories on your system/user path on initialization.