1. Create a "Service" and a "Provider" under the "gAppointments" sidebar menu.
2. Create a new form within Gravity Forms. Add a "Booking Calendar" field to this form.
3. Add the form to a post or page, using the block or shortcode.
4. Visit the page as any user (authenticated or not). Choose an appointment slot in the booking calendar field. Submit the form, and intercept the request (e.g. with BurpSuite).
5. In the POST request, change the value of input_6[time] to be blank (note that it may be called something other than input_6 on your site.
6. Then, change the value of input_6[cost] (same caveat on the naming) to '/><img src=x onerror=alert(/XSS/)>
7. Submit the request and disable interception. See the alert, indicating XSS.