I think the search facility of GravityView only references things in the form entry, not the view that is created, so the only way to get any content referenced in the search box is to make sure that content is present in the initial form and it's entry data. I do this by populating a hidden text paragraph field in the form. This then becomes another line in the form entry database that can be referenced in the search. You can then still place that content anywhere in a 'view' including a custom content box in the same way you add any form data into your views and it will be referenced by the search box.