The first two issues can be solved by adding

        Gst.init(null)


in a strategic place ;)

The last one

  g_object_class_find_property: assertion 'G_IS_OBJECT_CLASS (class)' failed

I don't know where that comes from, what line of js code triggers it. Perhaps you could ask some people familiar with gnome-shell or gjs how to debug this further (they probably have an environment where they can see more information and figure out what js call causes this). I put printfs around all other gst calls, so it might not be gstreamer-related.

this is the line makes this issue:
this.dmBus = this.deviceMonitor.get_bus();
and this is the LOG:
ott 29 07:52:17 ArchBox gnome-session[1043]: (gnome-shell:1120): GLib-GObject-CRITICAL **: g_object_class_find_property: assertion 'G_IS_OBJECT_CLASS (class)' failed
ott 29 07:52:18 ArchBox systemd-coredump[1788]: Process 1120 (gnome-shell) of user 1000 dumped core.
ott 29 07:52:18 ArchBox gnome-session-binary[1043]: WARNING: Application 'gnome-shell.desktop' killed by signal 11
ott 29 07:52:18 ArchBox gnome-session[1043]: gnome-session-binary[1043]: WARNING: Application 'gnome-shell.desktop' killed by signal 11

and this the coredump:
http://pastebin.com/vWCbvhCC

If I set a breakpoint in gdb, it doesn't even end up gst_device_monitor_get_bus(), so the issue is likely somewhere in the bindings layer.

However, if I do this:

  let device_monitor = new Gst.DeviceMonitor;

instead of

  let device_monitor = Gst.DeviceMonitor.new();

then it all seems to work fine at first glance:

------------------------------------------------------------
#!/usr/bin/gjs

const Gst = imports.gi.Gst;

Gst.init(null);
let device_monitor = new Gst.DeviceMonitor;
log("get_bus");
let bus = device_monitor.get_bus();
log("start");
device_monitor.start();
log("stop");
device_monitor.stop();
log("done");
------------------------------------------------------------

you're right, I used the function NEW as the documentation is shown as the only obj constructor in any case I'll open a bug report in the bindings section, thanks for your time :)

Alright, thanks. Closing then, but feel free to re-open and move it into the right product (or just clone it).