GravityForms usage of gform_datepicker_options_pre_init JS filter

gforms_js_filters.js

/* 
 * Create a JS file and upload it to your child theme
 * Assuming this directory structure: /wp-content/themes/<your_child_theme_folder/assets/js/gform_filters.js
 */
 
 //PHP Side - functions.php or some other way
 
 add_action( 'wp_enqueue_scripts', 'gform_datepicker_options_pre_init_filter' );
 function gform_datepicker_options_pre_init_filter() {
   //Add here any required conditionals to load below script conditionally
   wp_enqueue_script( 'child-gform', get_stylesheet_directory_uri() . '/assets/js/gform_filters.js', array('jquery'), null, true );
 }
 
 /*
  * JS Side - content of gform_filters.js file. 
  * Using https://docs.gravityforms.com/gform_datepicker_options_pre_init/#h-usage as reference
  * change formId and fieldId values depending on your own form
  * change disabledDays array, depending on the days you wish to disable
  */
  
document.addEventListener('DOMContentLoaded', function() {
 if( gform ) {
   gform.addFilter( 'gform_datepicker_options_pre_init', function( optionsObj, formId, fieldId ) {
      if ( formId == 12 && fieldId == 14 ) {
          var disabledDays = ['06/15/2014', '07/01/2014', '07/15/2014'];
          optionsObj.beforeShowDay = function(date) {
              var checkdate = jQuery.datepicker.formatDate('mm/dd/yy', date);
              return [disabledDays.indexOf(checkdate) == -1];
          };
      }
      return optionsObj;
  });
 }
});