Are fields unwritable?

We created a custom Gravity Forms form to let people submit their own testimonials on our website. The post content is created, but we cannot get the values inserted into the Client Name field. We’ve tried _testimonial_name, client_name, and testimonial_client as field names within WP Testimonials, but then nothing comes through on the interface. Are we choosing the wrong field name to send it to?

We’re using a name field, should it be a basic text field?

Thanks