Not sure if you are still looking for the solution, but…
First, you need to have a child theme, as you will be modifying functions.php. There are a number of good instructions on how to create a child theme. The reason to do this is to allow the theme to be updated by the developer without requiring you to go in afterwards and make your changes again.
Second, you will want to copy two functions from testimonials.php – found in the wp-content/plugins/gc-testimonials directory. These two are gct_form_shortcode() and gct_get_create_testimonial_form(). You will need to give them a new name – I prepend override_ onto the function names giving override_gct_form_shortcode() and override_gct_get_create_testimonial_form().
Inside overrid_gct_form_shortcode() you will want to find the line return gct_get_create_testimonial_form(… and change it to return override_gct_get_create_testimonial_form(… This will point it towards the function where you will make all of your modifications.
Make all modifications to the style and content of the form in override_gct_get_create_testimonial_form(). Any field that you wish to “remove”, make sure you change from <input type=”text” to <input type=”hidden”, DO NOT just delete the fields. You can also restyle the form in this section. Keep in mind that everything must be appended to the $html variable.
Finally, add the following two lines to function.php:
remove_shortcode(‘testimonial-form’);
add_shortcode(‘testimonial-form’, ‘override_gct_form_shortcode’);
This will point the [testimonial-form] shortcode to the new form.
