Hi,
You can create an option product, "Gift wrapping", and then add it to the products.
The fastest way to add it, is using phpMyAdmin. The options are added in the table "#__hikashop_product_related", the three columns present here are:
product_id product_related_id product_related_type
Just made a SQL request to add the option to all the products, the product_related_type value must be "options" and the gift wrapping id have to be in the column product_related_id.
Don't forget to backup before anything.