gfileutils.c makes heavy use of utility function set_file_error.
While that function work, none of the calls to it are correct.

Here's a sample call:

      set_file_error (error,
                      filename,
                      _("Failed to open file '%s': %s"));

The problem is that gettext (aka "_") can change errno at will.
Here's what that call should have looked like:

      int save_errno = errno;
      const char *fmt = _("Failed to open file '%s': %s");
      errno = save_errno;
      set_file_error (error, filename, fmt);

That looks crazy, so I would suggest to use N_(...) instead of _(...)
and have set_file_error call _.  For that to work right, format_error_message
would need to have the error number as an argument.