Add class to next and previous buttons

I'm using Twitter Bootstrap with Wordpress, and I use this filter to add a class to gforms submit buttons:

add_filter( 'gform_submit_button', 'theme_t_wp_submit_button', 10, 2 );
function theme_t_wp_submit_button( $button, $form ){

  return '<input class="btn btn-primary" type="submit" id="gform_submit_button_'.$form["id"].'" value="'. $form["button"]["text"] .'" />';
}

Is there a way to do something similar with next/previous buttons?