opencv’s android kit does NOT have gst or ffmpeg support by default.
(you have to build either locally (android!), then rebuild the opencv sdk with that)
besides, your gst code is entirely broken,
gst-launch is the binary, you would call on a (pc!) cmdline, this must not be part of the VideoCapture url
opening same thing twice …
if it really is plain mjpeg over http , you could still code your own capture
(it’s just a socket & multipart-form images; look it up…)