gw-populate-parent-entry-id-in-child.php

<?php
/**
 * Gravity Wiz // Nested Forms // Populate Parent Entry ID in Child Entry
 * https://gravitywiz.com/documentation/gravity-forms-nested-forms/
 *
 * Populate the parent entry ID into a field in the child entry upon submission of the parent form.
 */
add_action( 'gform_after_submission_123', function( $entry, $form ) { // Replace 123 with your parent form ID
  $parent_entry = new GPNF_Entry( $entry ); 
	$parent_id = rgar( $entry, 'id' );
	$child_entries = $parent_entry->get_child_entries( $nested_form_field_id );
	
	foreach( $child_entries as $child ){
		
		$child_id = rgar( $child, 'id' );
		
		GFAPI::update_entry_field( $child_id, 9, $parent_id ); // Replace 9 with the field ID that you want to populate in the child form
		
	}
	
}, 10, 2 );