Clarify dependencies for RG GStreamer backend

beetsplug/replaygain.py

@@ -280,7 +280,9 @@ class GStreamerBackend(object):
                 GObject.threads_init()
             Gst.init([sys.argv[0]])
         except:
-            raise FatalReplayGainError("GStreamer failed to initialize")
+            raise FatalReplayGainError(
+                "Failed to load GStreamer; check that python-gi is installed"
+            )
 
         self.GObject = GObject
         self.GLib = GLib

docs/plugins/replaygain.rst

@@ -23,8 +23,9 @@ GStreamer
 
 To use `GStreamer`_ for ReplayGain analysis, you will of course need to
 install GStreamer and plugins for compatibility with your audio files.
-You will need at least GStreamer 1.0.
+You will need at least GStreamer 1.0 and `PyGObject 3.x`_ (a.k.a. python-gi).
 
+.. _PyGObject 3.x: https://wiki.gnome.org/action/show/Projects/PyGObject
 .. _GStreamer: http://gstreamer.freedesktop.org/
 
 Then, enable the plugin (see :ref:`using-plugins`) and specify the GStreamer