Add the post ID to a custom field after_submission

<?php
// Insert the post ID into a custom field after the entry and post are created
// Help Scout 29933
// References:
// http://www.gravityhelp.com/documentation/page/Gform_after_submission
// http://codex.wordpress.org/Function_Reference/get_post_meta
// http://php.net/manual/en/function.str-replace.php
// http://codex.wordpress.org/Function_Reference/update_post_meta
// change the 411 here to your form ID
add_action('gform_after_submission_411', 'update_custom_field', 10, 2);
function update_custom_field($entry, $form) {
    // the post ID of the recently created post
    $pid = $entry['post_id'];
    // the meta key for the custom field we want to update
    $key = 'portfolio_item_description';
    // the placeholder text we are searching for
    $needle = 'PORTFOLIOPOSTID';
    // the content we are going to search within
    $haystack = get_post_meta($pid, $key, TRUE);
    // the modified content
    $new_description = str_replace($needle, $pid, $haystack);
    // insert the modified content back in the custom field
    update_post_meta($pid, $key, $new_description);
}