How to change “Gift Wrapper: ” label in email?

At this time there is no way to do it with settings.

In the future, ςε may create a field in the Settings in order to could change it from Dashboard…

Until then, you can use the wordpress api instead:

add_filter('gettext_gift-wrapping-for-woocommerce', function($translation, $text){
    if( $text == 'Gift wrapper' ){
        return 'my_text';
    }
    return $translation;
}, 10, 2);