add a hidden unique ID to the entry based on entry ID

<?php
// http://www.gravityhelp.com/forums/topic/entry-ids
// update the 93 here to your form ID
add_action('gform_after_submission_93', 'add_submission_id', 10, 2);
function add_submission_id($entry, $form) {
    global $wpdb;
    // update this to the number of the hidden field in your form
    $field_number = 4;
    // your submission ID format to be inserted into the hidden field
    $SubmissionID = 'A' . $entry['id'];
    // update the entry
    $wpdb->insert("{$wpdb->prefix}rg_lead_detail", array(
        'value'         => $SubmissionID,
        'field_number'  => $field_number,
        'lead_id'       => $entry['id'],
        'form_id'       => $entry['form_id']
    ));
}