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<?php
/**
 * Gravity Perks // GP Unique ID // Add Unique ID as Custom Field
 */
// update "123" to the ID of your form 
add_filter( 'gform_post_data_123', function( $post_data, $form, $entry ) {
	// update "my_custom_field" to the name of your custom field
	// update "4" to the ID of your Unique ID field
	$post_data['post_custom_fields']['my_custom_field'] = $entry[4];
	return $post_data;
}, 10, 3 );