Random Number Function

add_filter("gform_field_value_uuid", "get_unique");

function get_unique(){

    $prefix = "CF"; // update the prefix here

    do {
        $unique = mt_rand();
        $unique = substr($unique, 0, 8);
        $unique = $prefix . $unique;
    } while (!check_unique($unique));

    return $unique;
}

function check_unique($unique) {
    global $wpdb;

    $table = $wpdb->prefix . 'rg_lead_detail';
    $form_id = 28; // update to the form ID your unique id field belongs to
    $field_id = 2; // update to the field ID your unique id is being prepopulated in
    $result = $wpdb->get_var("SELECT value FROM $table WHERE form_id = '$form_id' AND field_number = '$field_id' AND value = '$unique'");

    if(empty($result))
        return true;

    return false;
}