The option --gui-mode has been removed with the following option:

https://github.com/prusa3d/Slic3r/commit/82bde51c2752656aed517d89356272de277408b4

As it causes issues to the Repetier Host application
https://github.com/prusa3d/Slic3r/issues/363

this option has been added with this commit for compatibility reasons.

slic3r.pl

@@ -35,6 +35,7 @@ my %cli_options = ();
         'ignore-nonexistent-config' => \$opt{ignore_nonexistent_config},
         'no-controller'         => \$opt{no_controller},
         'no-plater'             => \$opt{no_plater},
+        'gui-mode=s'            => \$opt{obsolete_ignore_this_option_gui_mode},
         'datadir=s'             => \$opt{datadir},
         'export-svg'            => \$opt{export_svg},
         'merge|m'               => \$opt{merge},