function example_preprocess_panels_pane(&$vars) {

  // add class per pane type
  switch ($vars['pane']->subtype) {
    case 'something-panel_pane_plan':
      $vars['classes_array'][] = 'gw-grid-1';
      break;
    case 'another-panel_pane_plan':
      $vars['classes_array'][] = 'gw-grid-2';
      break;
    case 'thisone-panel_pane_hub':
    case 'hellotest-panel_pane_hub':
      $vars['classes_array'][] = 'gw-grid-3';
      break;
  }

}