It appears that gdbus can output the error message:

  Error: Timeout was reached

so that when it is used in a script, the user cannot know where
this message comes from and is confused. See

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=729527

for instance.

When outputting an error message, a utility must always identifies
itself. Here, it should be:

  gdbus: Error: Timeout was reached

or

  gdbus error: Timeout was reached

(but since the error message comes from libgio, I suppose that the
former string is simpler to generate, e.g. with "gdbus: %s").

Actually, the line at fault appears to be in the gdbus utility
(gdbus-tool.c) rather than anything to do with the library:

      g_printerr (_("Error: %s\n"), error->message);

so either form would work.

All the g_printerr() calls in that file have the same problem of not
providing the context of "who said that?" when gdbus is used
noninteractively.

    S