Gstreamer should already be installed in pyhton3. In python3, type the following for checking:
import gi
gi.require_version('Gst', '1.0')
from gi.repository import Gst
print("Using Gst: {}.{}.{}.{}".format(*Gst.version()))
If this is ok (gstreamer is available from python3), the question may rather be: what is the opencv package used by python3 and does it match with where does my opencv install step as installed the python3 library, as said here.