I think, in the past (ubuntu pre-16.04) there were settings musicaudiosink and audiosink that could specify which JACK sink was to be used by gstreamer.
This changed the default behavior of gstreamer which was to connect to the first 2 sinks in JACK.
I think, this was the way it was in 0.1 gstreamer.
I've been able to find nothing discussing this for gstreamer 1 or for ubuntu 16.04.